Re: [PATCH] x86/uaccess: Fix 32-bit __get_user_asm_u64() when CC_HAS_ASM_GOTO_OUTPUT=y

[Naresh Kamboju]

On Mon, 13 Sept 2021 at 22:06, Will Deacon <will@xxxxxxxxxx> 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
> 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@xxxxxxxxxx>
> Cc: Bill Wendling <morbo@xxxxxxxxxx>
> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Reported-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> Fixes: 865c50e1d279 ("x86/uaccess: utilize CONFIG_CC_HAS_ASM_GOTO_OUTPUT")
> Signed-off-by: Will Deacon <will@xxxxxxxxxx>

This patch merged into Linux next (next-20210914) and the reported
problem has been fixed.

Tested-by: Linux Kernel Functional Testing <lkft@xxxxxxxxxx>

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