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FROMLIST: x86/uaccess: Fix 32-bit __get_user_asm_u64() when CC_HAS_ASM_GOTO_OUTPUT=y

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Commit 865c50e1d2 ("x86/uaccess: utilize CONFIG_CC_HAS_ASM_GOTO_OUTPUT")
added an optimised version of __get_user_asm() for x86 using 'asm goto'.

Like the non-optimised code, the 32-bit implementation of 64-bit get_user()
expands to a pair of 32-bit accesses. Unlike the non-optimised code, the
_original_ pointer is incremented to copy the high word instead of loading
through a new pointer explicitly constructed to point at a 32-bit type.
Consequently, if the pointer points at a 64-bit type then we end up
loading the wrong data for the upper 32-bits.

This was observed as a mount() failure in Android targetting i686 after
b0cfcdd9b9 ("d_path: make 'prepend()' fill up the buffer exactly on
overflow") because the call to copy_from_kernel_nofault() from
prepend_copy() ends up in __get_kernel_nofault() and casts the source
pointer to a 'u64 __user *'. An attempt to mount at "/debug_ramdisk"
therefore ends up failing trying to mount "/debumdismdisk".

Use the existing '__gu_ptr' source pointer to unsigned int for 32-bit
__get_user_asm_u64() instead of the original pointer.

Fixes: 865c50e1d2 ("x86/uaccess: utilize CONFIG_CC_HAS_ASM_GOTO_OUTPUT")
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Bill Wendling <morbo@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Reported-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20210913163547.5156-1-will@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: I652ecff43caf258bd0aa31e52fdd58f9461b9e46
This commit is contained in:
Will Deacon
2021-09-13 17:35:47 +01:00
committed by Greg Kroah-Hartman
parent 6e6c6d4507
commit 4d1eda771e
1 changed files with 2 additions and 2 deletions
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4
arch/x86/include/asm/uaccess.h
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@@ -301,8 +301,8 @@ do { \
unsigned int __gu_low, __gu_high; \
const unsigned int __user *__gu_ptr; \
__gu_ptr = (const void __user *)(ptr); \
__get_user_asm(__gu_low, ptr, "l", "=r", label); \
__get_user_asm(__gu_high, ptr+1, "l", "=r", label); \
__get_user_asm(__gu_low, __gu_ptr, "l", "=r", label); \
__get_user_asm(__gu_high, __gu_ptr+1, "l", "=r", label); \
(x) = ((unsigned long long)__gu_high << 32) | __gu_low; \
} while (0)
#else