CVE-2021-47430

In the Linux kernel, the following vulnerability has been resolved:
x86/entry: Clear X86_FEATURE_SMAP when CONFIG_X86_SMAP=n Commit
3c73b81a9164 (“x86/entry, selftests: Further improve user entry sanity
checks”) added a warning if AC is set when in the kernel. Commit
662a0221893a3d (“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. ]