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authorKees Cook <keescook@chromium.org>2021-09-01 12:42:08 -0700
committerThomas Bogendoerfer <tsbogend@alpha.franken.de>2022-02-23 13:08:30 +0100
commitfbb1d4b381b058ed60b39f1598532f559b441762 (patch)
treec9df02c5495ba6cce013a55e65dac2bbf989db45 /arch/mips/kernel
parentd1ca45f93c3f95d1590d60b012cf8fcf6db633ee (diff)
MIPS: Modernize READ_IMPLIES_EXEC
I'm doing some thread necromancy of https://lore.kernel.org/lkml/202007081624.82FA0CC1EA@keescook/ x86, arm64, and arm32 adjusted their READ_IMPLIES_EXEC logic to better align with the safer defaults and the interactions with other mappings, which I illustrated with this comment on x86: /* * 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-none | * 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 * "cannot be X" constraints in memory permission flags, as in * https://lkml.kernel.org/r/20190418055759.GA3155@mellanox.com * */ For MIPS, the "lacks NX" above is the "!cpu_has_rixi" check. On x86, we decided that the READ_IMPLIES_EXEC flag needed to reflect the expectations, not the architectural behavior due to bad interactions as noted above, as always returning "1" on non-NX hardware breaks some mappings. The other part of the issue is "what does the MIPS toolchain do for PT_GNU_STACK?" The answer seems to be "by default, include PT_GNU_STACK, but mark it executable" (likely due to concerns over non-NX hardware): $ mipsel-linux-gnu-gcc -o hello_world hello_world.c $ llvm-readelf -lW hellow_world | grep GNU_STACK GNU_STACK 0x000000 0x00000000 0x00000000 0x00000 0x00000 RWE 0x10 Given that older hardware doesn't support non-executable memory, it seems safe to make the "PT_GNU_STACK is absent" logic mean "assume non-executable", but this might break very old software running on modern MIPS. This situation matches the ia32-on-x86_64 logic x86 uses (which assumes needing READ_IMPLIES_EXEC in that situation). But modern toolchains on modern MIPS hardware should follow a safer default (assume NX stack). A follow-up to this change would be to switch the MIPS toolchain to emit a non-executable PT_GNU_STACK, as this seems to be unneeded. Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de> Cc: Tiezhu Yang <yangtiezhu@loongson.cn> Cc: Xuefeng Li <lixuefeng@loongson.cn> Cc: Juxin Gao <gaojuxin@loongson.cn> Cc: linux-mips@vger.kernel.org Signed-off-by: Kees Cook <keescook@chromium.org> Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Diffstat (limited to 'arch/mips/kernel')
-rw-r--r--arch/mips/kernel/elf.c16
1 files changed, 5 insertions, 11 deletions
diff --git a/arch/mips/kernel/elf.c b/arch/mips/kernel/elf.c
index 7b045d2a0b51..5582a4ca1e9e 100644
--- a/arch/mips/kernel/elf.c
+++ b/arch/mips/kernel/elf.c
@@ -328,16 +328,10 @@ void mips_set_personality_nan(struct arch_elf_state *state)
int mips_elf_read_implies_exec(void *elf_ex, int exstack)
{
- if (exstack != EXSTACK_DISABLE_X) {
- /* The binary doesn't request a non-executable stack */
- return 1;
- }
-
- if (!cpu_has_rixi) {
- /* The CPU doesn't support non-executable memory */
- return 1;
- }
-
- return 0;
+ /*
+ * Set READ_IMPLIES_EXEC only on non-NX systems that
+ * do not request a specific state via PT_GNU_STACK.
+ */
+ return (!cpu_has_rixi && exstack == EXSTACK_DEFAULT);
}
EXPORT_SYMBOL(mips_elf_read_implies_exec);