x86/entry: Clear X86_FEATURE_SMAP when CONFIG_X86_SMAP=n

Commit

  a42c4c2609bd ("x86/entry, selftests: Further improve user entry sanity checks")

added a warning if AC is set when in the kernel.

Commit

  8183a55d805152 ("x86/entry: Fix AC assertion")

changed the warning to only fire if the CPU supports SMAP.

However, the warning can still trigger on a machine that supports SMAP
but where it's disabled in the kernel config and when running the
syscall_nt selftest, for example:

  ------------[ cut here ]------------
  WARNING: CPU: 0 PID: 49 at irqentry_enter_from_user_mode
  CPU: 0 PID: 49 Comm: init Tainted: G                T 5.15.0-rc4+ #98 e6202628ee053b4f310759978284bd8bb0ce6905
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1ubuntu1 04/01/2014
  RIP: 0010:irqentry_enter_from_user_mode
  ...
  Call Trace:
   ? irqentry_enter
   ? exc_general_protection
   ? asm_exc_general_protection
   ? asm_exc_general_protectio

IS_ENABLED(CONFIG_X86_SMAP) could be added to the warning condition, but
even this would not be enough in case SMAP is disabled at boot time with
the "nosmap" parameter.

To be consistent with "nosmap" behaviour, clear X86_FEATURE_SMAP when
!CONFIG_X86_SMAP.

Found using entry-fuzz + satrandconfig.

 [ bp: Massage commit message. ]

Fixes: a42c4c2609bd ("x86/entry, selftests: Further improve user entry sanity checks")
Fixes: 8183a55d8051 ("x86/entry: Fix AC assertion")
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20211003223423.8666-1-vegard.nossum@oracle.com

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 0f8885949e8c4b602fefc2eaf918d11a2957c051..b3410f1ac21754c9ff94338a71da12e812d65686 100644 (file)
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -326,6 +326,7 @@ static __always_inline void setup_smap(struct cpuinfo_x86 *c)
 #ifdef CONFIG_X86_SMAP
                cr4_set_bits(X86_CR4_SMAP);
 #else
+               clear_cpu_cap(c, X86_FEATURE_SMAP);
                cr4_clear_bits(X86_CR4_SMAP);
 #endif
        }