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