From 9d9e435f3f2492bfd196acacb61cc9a9212d8170 Mon Sep 17 00:00:00 2001 From: Kees Cook Date: Thu, 26 Mar 2020 23:48:15 -0700 Subject: x86/elf: Add table to document READ_IMPLIES_EXEC Add a table to document the current behavior of READ_IMPLIES_EXEC in preparation for changing the behavior. Signed-off-by: Kees Cook Signed-off-by: Borislav Petkov Reviewed-by: Jason Gunthorpe Link: https://lkml.kernel.org/r/20200327064820.12602-2-keescook@chromium.org --- arch/x86/include/asm/elf.h | 19 +++++++++++++++++++ 1 file changed, 19 insertions(+) diff --git a/arch/x86/include/asm/elf.h b/arch/x86/include/asm/elf.h index 69c0f892e310..ee459d4c3b45 100644 --- a/arch/x86/include/asm/elf.h +++ b/arch/x86/include/asm/elf.h @@ -281,6 +281,25 @@ extern u32 elf_hwcap2; /* * An executable for which elf_read_implies_exec() returns TRUE will * have the READ_IMPLIES_EXEC personality flag set automatically. + * + * The decision process for determining the results are: + * + *              CPU: | lacks NX*  | has NX, ia32     | has NX, x86_64 | + * ELF:              |            |                  |                | + * ---------------------|------------|------------------|----------------| + * missing PT_GNU_STACK | exec-all   | exec-all         | exec-all       | + * PT_GNU_STACK == RWX  | exec-all   | exec-all         | exec-all       | + * PT_GNU_STACK == RW   | exec-none  | exec-none        | exec-none      | + * + * exec-all : all PROT_READ user mappings are executable, except when + * backed by files on a noexec-filesystem. + * exec-none : only PROT_EXEC user mappings are executable. + * + * *this column has no architectural effect: NX markings are ignored by + * hardware, but may have behavioral effects when "wants X" collides with + * "cannot be X" constraints in memory permission flags, as in + * https://lkml.kernel.org/r/20190418055759.GA3155@mellanox.com + * */ #define elf_read_implies_exec(ex, executable_stack) \ (executable_stack != EXSTACK_DISABLE_X) -- cgit From 122306117afe4ba202b5e57c61dfbeffc5c41387 Mon Sep 17 00:00:00 2001 From: Kees Cook Date: Thu, 26 Mar 2020 23:48:16 -0700 Subject: x86/elf: Split READ_IMPLIES_EXEC from executable PT_GNU_STACK The READ_IMPLIES_EXEC workaround was designed for old toolchains that lacked the ELF PT_GNU_STACK marking under the assumption that toolchains that couldn't specify executable permission flags for the stack may not know how to do it correctly for any memory region. This logic is sensible for having ancient binaries coexist in a system with possibly NX memory, but was implemented in a way that equated having a PT_GNU_STACK marked executable as being as "broken" as lacking the PT_GNU_STACK marking entirely. Things like unmarked assembly and stack trampolines may cause PT_GNU_STACK to need an executable bit, but they do not imply all mappings must be executable. This confusion has led to situations where modern programs with explicitly marked executable stacks are forced into the READ_IMPLIES_EXEC state when no such thing is needed. (And leads to unexpected failures when mmap()ing regions of device driver memory that wish to disallow VM_EXEC[1].) In looking for other reasons for the READ_IMPLIES_EXEC behavior, Jann Horn noted that glibc thread stacks have always been marked RWX (until 2003 when they started tracking the PT_GNU_STACK flag instead[2]). And musl doesn't support executable stacks at all[3]. As such, no breakage for multithreaded applications is expected from this change. [1] https://lkml.kernel.org/r/20190418055759.GA3155@mellanox.com [2] https://sourceware.org/git/?p=glibc.git;a=commitdiff;h=54ee14b3882 [3] https://lkml.kernel.org/r/20190423192534.GN23599@brightrain.aerifal.cx Suggested-by: Hector Marco-Gisbert Signed-off-by: Kees Cook Signed-off-by: Borislav Petkov Reviewed-by: Jason Gunthorpe Link: https://lkml.kernel.org/r/20200327064820.12602-3-keescook@chromium.org --- arch/x86/include/asm/elf.h | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-) diff --git a/arch/x86/include/asm/elf.h b/arch/x86/include/asm/elf.h index ee459d4c3b45..397a1c74433e 100644 --- a/arch/x86/include/asm/elf.h +++ b/arch/x86/include/asm/elf.h @@ -288,12 +288,13 @@ extern u32 elf_hwcap2; * ELF:              |            |                  |                | * ---------------------|------------|------------------|----------------| * missing PT_GNU_STACK | exec-all   | exec-all         | exec-all       | - * PT_GNU_STACK == RWX  | exec-all   | exec-all         | exec-all       | + * PT_GNU_STACK == RWX  | exec-stack | exec-stack       | exec-stack     | * PT_GNU_STACK == RW   | exec-none  | exec-none        | exec-none      | * * exec-all : all PROT_READ user mappings are executable, except when * backed by files on a noexec-filesystem. * exec-none : only PROT_EXEC user mappings are executable. + * exec-stack: only the stack and PROT_EXEC user mappings are executable. * * *this column has no architectural effect: NX markings are ignored by * hardware, but may have behavioral effects when "wants X" collides with @@ -302,7 +303,7 @@ extern u32 elf_hwcap2; * */ #define elf_read_implies_exec(ex, executable_stack) \ - (executable_stack != EXSTACK_DISABLE_X) + (executable_stack == EXSTACK_DEFAULT) struct task_struct; -- cgit From 9fccc5c0c99f238aa1b0460fccbdb30a887e7036 Mon Sep 17 00:00:00 2001 From: Kees Cook Date: Thu, 26 Mar 2020 23:48:17 -0700 Subject: x86/elf: Disable automatic READ_IMPLIES_EXEC on 64-bit With modern x86 64-bit environments, there should never be a need for automatic READ_IMPLIES_EXEC, as the architecture is intended to always be execute-bit aware (as in, the default memory protection should be NX unless a region explicitly requests to be executable). There were very old x86_64 systems that lacked the NX bit, but for those, the NX bit is, obviously, unenforceable, so these changes should have no impact on them. Suggested-by: Hector Marco-Gisbert Signed-off-by: Kees Cook Signed-off-by: Borislav Petkov Reviewed-by: Jason Gunthorpe Link: https://lkml.kernel.org/r/20200327064820.12602-4-keescook@chromium.org --- arch/x86/include/asm/elf.h | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/arch/x86/include/asm/elf.h b/arch/x86/include/asm/elf.h index 397a1c74433e..452beed7892b 100644 --- a/arch/x86/include/asm/elf.h +++ b/arch/x86/include/asm/elf.h @@ -287,7 +287,7 @@ extern u32 elf_hwcap2; *              CPU: | lacks NX*  | has NX, ia32     | has NX, x86_64 | * ELF:              |            |                  |                | * ---------------------|------------|------------------|----------------| - * missing PT_GNU_STACK | exec-all   | exec-all         | exec-all       | + * missing PT_GNU_STACK | exec-all   | exec-all         | exec-none      | * PT_GNU_STACK == RWX  | exec-stack | exec-stack       | exec-stack     | * PT_GNU_STACK == RW   | exec-none  | exec-none        | exec-none      | * @@ -303,7 +303,7 @@ extern u32 elf_hwcap2; * */ #define elf_read_implies_exec(ex, executable_stack) \ - (executable_stack == EXSTACK_DEFAULT) + (mmap_is_ia32() && executable_stack == EXSTACK_DEFAULT) struct task_struct; -- cgit From 78066055b08096ed3282c027de9d9e137f9a1580 Mon Sep 17 00:00:00 2001 From: Kees Cook Date: Thu, 26 Mar 2020 23:48:18 -0700 Subject: arm32/64/elf: Add tables to document READ_IMPLIES_EXEC Add tables to document the current behavior of READ_IMPLIES_EXEC in preparation for changing the behavior for both arm64 and arm. Signed-off-by: Kees Cook Signed-off-by: Borislav Petkov Reviewed-by: Jason Gunthorpe Reviewed-by: Catalin Marinas Link: https://lkml.kernel.org/r/20200327064820.12602-5-keescook@chromium.org --- arch/arm/kernel/elf.c | 24 +++++++++++++++++++++--- arch/arm64/include/asm/elf.h | 20 ++++++++++++++++++++ 2 files changed, 41 insertions(+), 3 deletions(-) diff --git a/arch/arm/kernel/elf.c b/arch/arm/kernel/elf.c index 182422981386..5ccd4aced6cc 100644 --- a/arch/arm/kernel/elf.c +++ b/arch/arm/kernel/elf.c @@ -78,9 +78,27 @@ void elf_set_personality(const struct elf32_hdr *x) EXPORT_SYMBOL(elf_set_personality); /* - * Set READ_IMPLIES_EXEC if: - * - the binary requires an executable stack - * - we're running on a CPU which doesn't support NX. + * An executable for which elf_read_implies_exec() returns TRUE will + * have the READ_IMPLIES_EXEC personality flag set automatically. + * + * The decision process for determining the results are: + * + *              CPU: | lacks NX*  | has NX | + * ELF:              |            |           | + * ---------------------|------------|------------| + * missing PT_GNU_STACK | exec-all   | exec-all  | + * PT_GNU_STACK == RWX  | exec-all   | exec-all  | + * PT_GNU_STACK == RW   | exec-all  | exec-none | + * + * exec-all : all PROT_READ user mappings are executable, except when + * backed by files on a noexec-filesystem. + * exec-none : only PROT_EXEC user mappings are executable. + * + * *this column has no architectural effect: NX markings are ignored by + * hardware, but may have behavioral effects when "wants X" collides with + * "cannot be X" constraints in memory permission flags, as in + * https://lkml.kernel.org/r/20190418055759.GA3155@mellanox.com + * */ int arm_elf_read_implies_exec(int executable_stack) { diff --git a/arch/arm64/include/asm/elf.h b/arch/arm64/include/asm/elf.h index b618017205a3..986ecf41fc0f 100644 --- a/arch/arm64/include/asm/elf.h +++ b/arch/arm64/include/asm/elf.h @@ -96,6 +96,26 @@ */ #define elf_check_arch(x) ((x)->e_machine == EM_AARCH64) +/* + * An executable for which elf_read_implies_exec() returns TRUE will + * have the READ_IMPLIES_EXEC personality flag set automatically. + * + * The decision process for determining the results are: + * + *              CPU*: | arm32    | arm64 | + * ELF:              |            |            | + * ---------------------|------------|------------| + * missing PT_GNU_STACK | exec-all   | exec-all   | + * PT_GNU_STACK == RWX  | exec-all   | exec-all   | + * PT_GNU_STACK == RW   | exec-none | exec-none | + * + * exec-all : all PROT_READ user mappings are executable, except when + * backed by files on a noexec-filesystem. + * exec-none : only PROT_EXEC user mappings are executable. + * + * *all arm64 CPUs support NX, so there is no "lacks NX" column. + * + */ #define elf_read_implies_exec(ex,stk) (stk != EXSTACK_DISABLE_X) #define CORE_DUMP_USE_REGSET -- cgit From eaf3f9e61887332d5097dbf0b327b8377546adc5 Mon Sep 17 00:00:00 2001 From: Kees Cook Date: Thu, 26 Mar 2020 23:48:19 -0700 Subject: arm32/64/elf: Split READ_IMPLIES_EXEC from executable PT_GNU_STACK The READ_IMPLIES_EXEC work-around was designed for old toolchains that lacked the ELF PT_GNU_STACK marking under the assumption that toolchains that couldn't specify executable permission flags for the stack may not know how to do it correctly for any memory region. This logic is sensible for having ancient binaries coexist in a system with possibly NX memory, but was implemented in a way that equated having a PT_GNU_STACK marked executable as being as "broken" as lacking the PT_GNU_STACK marking entirely. Things like unmarked assembly and stack trampolines may cause PT_GNU_STACK to need an executable bit, but they do not imply all mappings must be executable. This confusion has led to situations where modern programs with explicitly marked executable stack are forced into the READ_IMPLIES_EXEC state when no such thing is needed. (And leads to unexpected failures when mmap()ing regions of device driver memory that wish to disallow VM_EXEC[1].) In looking for other reasons for the READ_IMPLIES_EXEC behavior, Jann Horn noted that glibc thread stacks have always been marked RWX (until 2003 when they started tracking the PT_GNU_STACK flag instead[2]). And musl doesn't support executable stacks at all[3]. As such, no breakage for multithreaded applications is expected from this change. This changes arm32 and arm64 compat together, to keep behavior the same. [1] https://lkml.kernel.org/r/20190418055759.GA3155@mellanox.com [2] https://sourceware.org/git/?p=glibc.git;a=commitdiff;h=54ee14b3882 [3] https://lkml.kernel.org/r/20190423192534.GN23599@brightrain.aerifal.cx Suggested-by: Hector Marco-Gisbert Signed-off-by: Kees Cook Signed-off-by: Borislav Petkov Reviewed-by: Jason Gunthorpe Reviewed-by: Catalin Marinas Link: https://lkml.kernel.org/r/20200327064820.12602-6-keescook@chromium.org --- arch/arm/kernel/elf.c | 5 +++-- arch/arm64/include/asm/elf.h | 5 +++-- 2 files changed, 6 insertions(+), 4 deletions(-) diff --git a/arch/arm/kernel/elf.c b/arch/arm/kernel/elf.c index 5ccd4aced6cc..254ab7138c85 100644 --- a/arch/arm/kernel/elf.c +++ b/arch/arm/kernel/elf.c @@ -87,12 +87,13 @@ EXPORT_SYMBOL(elf_set_personality); * ELF:              |            |           | * ---------------------|------------|------------| * missing PT_GNU_STACK | exec-all   | exec-all  | - * PT_GNU_STACK == RWX  | exec-all   | exec-all  | + * PT_GNU_STACK == RWX  | exec-all   | exec-stack | * PT_GNU_STACK == RW   | exec-all  | exec-none | * * exec-all : all PROT_READ user mappings are executable, except when * backed by files on a noexec-filesystem. * exec-none : only PROT_EXEC user mappings are executable. + * exec-stack: only the stack and PROT_EXEC user mappings are executable. * * *this column has no architectural effect: NX markings are ignored by * hardware, but may have behavioral effects when "wants X" collides with @@ -102,7 +103,7 @@ EXPORT_SYMBOL(elf_set_personality); */ int arm_elf_read_implies_exec(int executable_stack) { - if (executable_stack != EXSTACK_DISABLE_X) + if (executable_stack == EXSTACK_DEFAULT) return 1; if (cpu_architecture() < CPU_ARCH_ARMv6) return 1; diff --git a/arch/arm64/include/asm/elf.h b/arch/arm64/include/asm/elf.h index 986ecf41fc0f..0074e9fd6431 100644 --- a/arch/arm64/include/asm/elf.h +++ b/arch/arm64/include/asm/elf.h @@ -106,17 +106,18 @@ * ELF:              |            |            | * ---------------------|------------|------------| * missing PT_GNU_STACK | exec-all   | exec-all   | - * PT_GNU_STACK == RWX  | exec-all   | exec-all   | + * PT_GNU_STACK == RWX  | exec-stack | exec-stack | * PT_GNU_STACK == RW   | exec-none | exec-none | * * exec-all : all PROT_READ user mappings are executable, except when * backed by files on a noexec-filesystem. * exec-none : only PROT_EXEC user mappings are executable. + * exec-stack: only the stack and PROT_EXEC user mappings are executable. * * *all arm64 CPUs support NX, so there is no "lacks NX" column. * */ -#define elf_read_implies_exec(ex,stk) (stk != EXSTACK_DISABLE_X) +#define elf_read_implies_exec(ex, stk) (stk == EXSTACK_DEFAULT) #define CORE_DUMP_USE_REGSET #define ELF_EXEC_PAGESIZE PAGE_SIZE -- cgit From 6e0d6ac5f3d9d90271899f6d340872360fe1caee Mon Sep 17 00:00:00 2001 From: Kees Cook Date: Thu, 26 Mar 2020 23:48:20 -0700 Subject: arm64/elf: Disable automatic READ_IMPLIES_EXEC for 64-bit address spaces With arm64 64-bit environments, there should never be a need for automatic READ_IMPLIES_EXEC, as the architecture has always been execute-bit aware (as in, the default memory protection should be NX unless a region explicitly requests to be executable). Suggested-by: Hector Marco-Gisbert Signed-off-by: Kees Cook Signed-off-by: Borislav Petkov Reviewed-by: Jason Gunthorpe Reviewed-by: Catalin Marinas Link: https://lkml.kernel.org/r/20200327064820.12602-7-keescook@chromium.org --- arch/arm64/include/asm/elf.h | 4 ++-- fs/compat_binfmt_elf.c | 5 +++++ 2 files changed, 7 insertions(+), 2 deletions(-) diff --git a/arch/arm64/include/asm/elf.h b/arch/arm64/include/asm/elf.h index 0074e9fd6431..0e7df6f1eb7a 100644 --- a/arch/arm64/include/asm/elf.h +++ b/arch/arm64/include/asm/elf.h @@ -105,7 +105,7 @@ *              CPU*: | arm32    | arm64 | * ELF:              |            |            | * ---------------------|------------|------------| - * missing PT_GNU_STACK | exec-all   | exec-all   | + * missing PT_GNU_STACK | exec-all   | exec-none  | * PT_GNU_STACK == RWX  | exec-stack | exec-stack | * PT_GNU_STACK == RW   | exec-none | exec-none | * @@ -117,7 +117,7 @@ * *all arm64 CPUs support NX, so there is no "lacks NX" column. * */ -#define elf_read_implies_exec(ex, stk) (stk == EXSTACK_DEFAULT) +#define compat_elf_read_implies_exec(ex, stk) (stk == EXSTACK_DEFAULT) #define CORE_DUMP_USE_REGSET #define ELF_EXEC_PAGESIZE PAGE_SIZE diff --git a/fs/compat_binfmt_elf.c b/fs/compat_binfmt_elf.c index aaad4ca1217e..3068d57436b3 100644 --- a/fs/compat_binfmt_elf.c +++ b/fs/compat_binfmt_elf.c @@ -113,6 +113,11 @@ #define arch_setup_additional_pages compat_arch_setup_additional_pages #endif +#ifdef compat_elf_read_implies_exec +#undef elf_read_implies_exec +#define elf_read_implies_exec compat_elf_read_implies_exec +#endif + /* * Rename a few of the symbols that binfmt_elf.c will define. * These are all local so the names don't really matter, but it -- cgit