From a69ae291e1cc2d08ae77c2029579c59c9bde5061 Mon Sep 17 00:00:00 2001

From: Will Deacon <will@kernel.org>

Date: Mon, 13 Sep 2021 17:35:47 +0100

Subject: [PATCH] x86/uaccess: Fix 32-bit __get_user_asm_u64() when

 CC_HAS_ASM_GOTO_OUTPUT=y

Git-commit: a69ae291e1cc2d08ae77c2029579c59c9bde5061

References: git-fixes stable-5.14.7

Patch-mainline: v5.15-rc2

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 targeting 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: Bill Wendling <morbo@google.com>

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>

Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

Tested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Signed-off-by: Oliver Neukum <oneukum@suse.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

-- 

2.26.2