Messages in this thread | | | From | Nick Desaulniers <> | Date | Mon, 13 Sep 2021 09:45:51 -0700 | Subject | Re: [PATCH] x86/uaccess: Fix 32-bit __get_user_asm_u64() when CC_HAS_ASM_GOTO_OUTPUT=y |
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On Mon, Sep 13, 2021 at 9:36 AM Will Deacon <will@kernel.org> wrote: > > Commit 865c50e1d279 ("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
s/targetting/targeting/
> b0cfcdd9b967 ("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. > > 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> > Fixes: 865c50e1d279 ("x86/uaccess: utilize CONFIG_CC_HAS_ASM_GOTO_OUTPUT") > Signed-off-by: Will Deacon <will@kernel.org>
Sorry I missed this; I think -Wunused-variable would have helped here. Thanks for debugging+fixing.
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
> --- > arch/x86/include/asm/uaccess.h | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h > index c9fa7be3df82..5c95d242f38d 100644 > --- a/arch/x86/include/asm/uaccess.h > +++ b/arch/x86/include/asm/uaccess.h > @@ -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 > -- > 2.33.0.309.g3052b89438-goog >
-- Thanks, ~Nick Desaulniers
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