[PATCH] x86: also disable FSRM if ERMS is disabled

[Daniel Verkamp]

In the "Fast Short REP MOVSB" path of memmove, if we take the path where
the FSRM flag is enabled but the ERMS flag is not, there is no longer a
check for length >= 0x20 (both alternatives will be replaced with NOPs).
If a memmove() requiring a forward copy of less than 0x20 bytes happens
in this case, the `sub $0x20, %rdx` will cause the length to roll around
to a huge value and the copy will eventually hit a page fault.

This is not intended to happen, as the comment above the alternatives
mentions "FSRM implies ERMS".

However, there is a check in early_init_intel() that can disable ERMS,
so we should also be disabling FSRM in this path to maintain correctness
of the memmove() optimization.

Cc: stable@xxxxxxxxxxxxxxx
Fixes: f444a5ff95dc ("x86/cpufeatures: Add support for fast short REP; MOVSB")
Signed-off-by: Daniel Verkamp <dverkamp@xxxxxxxxxxxx>
---
 arch/x86/kernel/cpu/intel.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
index 2d7ea5480ec3..71b412f820c7 100644
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -328,6 +328,7 @@ static void early_init_intel(struct cpuinfo_x86 *c)
 			pr_info("Disabled fast string operations\n");
 			setup_clear_cpu_cap(X86_FEATURE_REP_GOOD);
 			setup_clear_cpu_cap(X86_FEATURE_ERMS);
+			setup_clear_cpu_cap(X86_FEATURE_FSRM);
 		}
 	}
 
-- 
2.37.3.998.g577e59143f-goog