From: Russell King rmk+kernel@armlinux.org.uk
Commit a3c0f84765bb429ba0fd23de1c57b5e1591c9389 upstream.
Spectre variant 1 attacks are about this sequence of pseudo-code:
index = load(user-manipulated pointer); access(base + index * stride);
In order for the cache side-channel to work, the access() must me made to memory which userspace can detect whether cache lines have been loaded. On 32-bit ARM, this must be either user accessible memory, or a kernel mapping of that same user accessible memory.
The problem occurs when the load() speculatively loads privileged data, and the subsequent access() is made to user accessible memory.
Any load() which makes use of a user-maniplated pointer is a potential problem if the data it has loaded is used in a subsequent access. This also applies for the access() if the data loaded by that access is used by a subsequent access.
Harden the get_user() accessors against Spectre attacks by forcing out of bounds addresses to a NULL pointer. This prevents get_user() being used as the load() step above. As a side effect, put_user() will also be affected even though it isn't implicated.
Also harden copy_from_user() by redoing the bounds check within the arm_copy_from_user() code, and NULLing the pointer if out of bounds.
Acked-by: Mark Rutland mark.rutland@arm.com Signed-off-by: Russell King rmk+kernel@armlinux.org.uk Signed-off-by: David A. Long dave.long@linaro.org --- arch/arm/include/asm/assembler.h | 4 ++++ arch/arm/lib/copy_from_user.S | 9 +++++++++ 2 files changed, 13 insertions(+)
diff --git a/arch/arm/include/asm/assembler.h b/arch/arm/include/asm/assembler.h index 189f3b42baea..e616f61f859d 100644 --- a/arch/arm/include/asm/assembler.h +++ b/arch/arm/include/asm/assembler.h @@ -458,6 +458,10 @@ THUMB( orr \reg , \reg , #PSR_T_BIT ) adds \tmp, \addr, #\size - 1 sbcccs \tmp, \tmp, \limit bcs \bad +#ifdef CONFIG_CPU_SPECTRE + movcs \addr, #0 + csdb +#endif #endif .endm
diff --git a/arch/arm/lib/copy_from_user.S b/arch/arm/lib/copy_from_user.S index 7a4b06049001..a826df3d3814 100644 --- a/arch/arm/lib/copy_from_user.S +++ b/arch/arm/lib/copy_from_user.S @@ -90,6 +90,15 @@ .text
ENTRY(arm_copy_from_user) +#ifdef CONFIG_CPU_SPECTRE + get_thread_info r3 + ldr r3, [r3, #TI_ADDR_LIMIT] + adds ip, r1, r2 @ ip=addr+size + sub r3, r3, #1 @ addr_limit - 1 + cmpcc ip, r3 @ if (addr+size > addr_limit - 1) + movcs r1, #0 @ addr = NULL + csdb +#endif
#include "copy_template.S"