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authorLinus Torvalds <torvalds@linux-foundation.org>2019-02-25 09:10:51 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2019-02-25 09:10:51 -0800
commit53a41cb7ed381edee91029cdcabe9b3250f43f4d (patch)
tree9c22c265fcde82972e9fa0333ee27e612e3a1491
parent5908e6b738e3357af42c10e1183753c70a0117a9 (diff)
Revert "x86/fault: BUG() when uaccess helpers fault on kernel addresses"
This reverts commit 9da3f2b74054406f87dff7101a569217ffceb29b. It was well-intentioned, but wrong. Overriding the exception tables for instructions for random reasons is just wrong, and that is what the new code did. It caused problems for tracing, and it caused problems for strncpy_from_user(), because the new checks made perfectly valid use cases break, rather than catch things that did bad things. Unchecked user space accesses are a problem, but that's not a reason to add invalid checks that then people have to work around with silly flags (in this case, that 'kernel_uaccess_faults_ok' flag, which is just an odd way to say "this commit was wrong" and was sprinked into random places to hide the wrongness). The real fix to unchecked user space accesses is to get rid of the special "let's not check __get_user() and __put_user() at all" logic. Make __{get|put}_user() be just aliases to the regular {get|put}_user() functions, and make it impossible to access user space without having the proper checks in places. The raison d'ĂȘtre of the special double-underscore versions used to be that the range check was expensive, and if you did multiple user accesses, you'd do the range check up front (like the signal frame handling code, for example). But SMAP (on x86) and PAN (on ARM) have made that optimization pointless, because the _real_ expense is the "set CPU flag to allow user space access". Do let's not break the valid cases to catch invalid cases that shouldn't even exist. Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Kees Cook <keescook@chromium.org> Cc: Tobin C. Harding <tobin@kernel.org> Cc: Borislav Petkov <bp@alien8.de> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Andy Lutomirski <luto@kernel.org> Cc: Jann Horn <jannh@google.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
-rw-r--r--arch/x86/mm/extable.c58
-rw-r--r--fs/namespace.c2
-rw-r--r--include/linux/sched.h6
-rw-r--r--mm/maccess.c6
4 files changed, 0 insertions, 72 deletions
diff --git a/arch/x86/mm/extable.c b/arch/x86/mm/extable.c
index 6521134057e8..856fa409c536 100644
--- a/arch/x86/mm/extable.c
+++ b/arch/x86/mm/extable.c
@@ -117,67 +117,11 @@ __visible bool ex_handler_fprestore(const struct exception_table_entry *fixup,
}
EXPORT_SYMBOL_GPL(ex_handler_fprestore);
-/* Helper to check whether a uaccess fault indicates a kernel bug. */
-static bool bogus_uaccess(struct pt_regs *regs, int trapnr,
- unsigned long fault_addr)
-{
- /* This is the normal case: #PF with a fault address in userspace. */
- if (trapnr == X86_TRAP_PF && fault_addr < TASK_SIZE_MAX)
- return false;
-
- /*
- * This code can be reached for machine checks, but only if the #MC
- * handler has already decided that it looks like a candidate for fixup.
- * This e.g. happens when attempting to access userspace memory which
- * the CPU can't access because of uncorrectable bad memory.
- */
- if (trapnr == X86_TRAP_MC)
- return false;
-
- /*
- * There are two remaining exception types we might encounter here:
- * - #PF for faulting accesses to kernel addresses
- * - #GP for faulting accesses to noncanonical addresses
- * Complain about anything else.
- */
- if (trapnr != X86_TRAP_PF && trapnr != X86_TRAP_GP) {
- WARN(1, "unexpected trap %d in uaccess\n", trapnr);
- return false;
- }
-
- /*
- * This is a faulting memory access in kernel space, on a kernel
- * address, in a usercopy function. This can e.g. be caused by improper
- * use of helpers like __put_user and by improper attempts to access
- * userspace addresses in KERNEL_DS regions.
- * The one (semi-)legitimate exception are probe_kernel_{read,write}(),
- * which can be invoked from places like kgdb, /dev/mem (for reading)
- * and privileged BPF code (for reading).
- * The probe_kernel_*() functions set the kernel_uaccess_faults_ok flag
- * to tell us that faulting on kernel addresses, and even noncanonical
- * addresses, in a userspace accessor does not necessarily imply a
- * kernel bug, root might just be doing weird stuff.
- */
- if (current->kernel_uaccess_faults_ok)
- return false;
-
- /* This is bad. Refuse the fixup so that we go into die(). */
- if (trapnr == X86_TRAP_PF) {
- pr_emerg("BUG: pagefault on kernel address 0x%lx in non-whitelisted uaccess\n",
- fault_addr);
- } else {
- pr_emerg("BUG: GPF in non-whitelisted uaccess (non-canonical address?)\n");
- }
- return true;
-}
-
__visible bool ex_handler_uaccess(const struct exception_table_entry *fixup,
struct pt_regs *regs, int trapnr,
unsigned long error_code,
unsigned long fault_addr)
{
- if (bogus_uaccess(regs, trapnr, fault_addr))
- return false;
regs->ip = ex_fixup_addr(fixup);
return true;
}
@@ -188,8 +132,6 @@ __visible bool ex_handler_ext(const struct exception_table_entry *fixup,
unsigned long error_code,
unsigned long fault_addr)
{
- if (bogus_uaccess(regs, trapnr, fault_addr))
- return false;
/* Special hack for uaccess_err */
current->thread.uaccess_err = 1;
regs->ip = ex_fixup_addr(fixup);
diff --git a/fs/namespace.c b/fs/namespace.c
index a677b59efd74..678ef175d63a 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2698,7 +2698,6 @@ static long exact_copy_from_user(void *to, const void __user * from,
if (!access_ok(from, n))
return n;
- current->kernel_uaccess_faults_ok++;
while (n) {
if (__get_user(c, f)) {
memset(t, 0, n);
@@ -2708,7 +2707,6 @@ static long exact_copy_from_user(void *to, const void __user * from,
f++;
n--;
}
- current->kernel_uaccess_faults_ok--;
return n;
}
diff --git a/include/linux/sched.h b/include/linux/sched.h
index bba3afb4e9bf..f9b43c989577 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -739,12 +739,6 @@ struct task_struct {
unsigned use_memdelay:1;
#endif
- /*
- * May usercopy functions fault on kernel addresses?
- * This is not just a single bit because this can potentially nest.
- */
- unsigned int kernel_uaccess_faults_ok;
-
unsigned long atomic_flags; /* Flags requiring atomic access. */
struct restart_block restart_block;
diff --git a/mm/maccess.c b/mm/maccess.c
index f3416632e5a4..ec00be51a24f 100644
--- a/mm/maccess.c
+++ b/mm/maccess.c
@@ -30,10 +30,8 @@ long __probe_kernel_read(void *dst, const void *src, size_t size)
set_fs(KERNEL_DS);
pagefault_disable();
- current->kernel_uaccess_faults_ok++;
ret = __copy_from_user_inatomic(dst,
(__force const void __user *)src, size);
- current->kernel_uaccess_faults_ok--;
pagefault_enable();
set_fs(old_fs);
@@ -60,9 +58,7 @@ long __probe_kernel_write(void *dst, const void *src, size_t size)
set_fs(KERNEL_DS);
pagefault_disable();
- current->kernel_uaccess_faults_ok++;
ret = __copy_to_user_inatomic((__force void __user *)dst, src, size);
- current->kernel_uaccess_faults_ok--;
pagefault_enable();
set_fs(old_fs);
@@ -98,13 +94,11 @@ long strncpy_from_unsafe(char *dst, const void *unsafe_addr, long count)
set_fs(KERNEL_DS);
pagefault_disable();
- current->kernel_uaccess_faults_ok++;
do {
ret = __get_user(*dst++, (const char __user __force *)src++);
} while (dst[-1] && ret == 0 && src - unsafe_addr < count);
- current->kernel_uaccess_faults_ok--;
dst[-1] = '\0';
pagefault_enable();
set_fs(old_fs);