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Kernel: Fix incorrect SFMASK MSR value clobbering reserved bits

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Also improve the comments around that initialisation code.
This commit is contained in:
Owen Smith 2021-12-29 23:27:44 +00:00 • committed by Andreas Kling
parent d364c8e352
commit 3f2b70382e
1 changed files with 9 additions and 5 deletions
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14
Kernel/Arch/x86/common/Processor.cpp
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@ -223,7 +223,7 @@ UNMAP_AFTER_INIT void Processor::cpu_setup()
}
#if ARCH(X86_64)
// x86_64 processors must have the syscall feature.
// x86_64 processors must support the syscall feature.
VERIFY(has_feature(CPUFeature::SYSCALL));
MSR efer_msr(MSR_EFER);
efer_msr.set(efer_msr.get() | 1u);
@ -236,12 +236,16 @@ UNMAP_AFTER_INIT void Processor::cpu_setup()
MSR star_msr(MSR_STAR);
star_msr.set(star);
// Write the syscall entry point to the LSTAR MSR, and write the SFMASK MSR to clear rflags upon entry.
// The userspace rflags will be preserved in r11.
// Write the syscall entry point to the LSTAR MSR.
MSR lstar_msr(MSR_LSTAR);
MSR sfmask_msr(MSR_SFMASK);
lstar_msr.set(reinterpret_cast<u64>(&syscall_entry));
sfmask_msr.set(~0x2);
// Write the SFMASK MSR. This MSR controls which bits of rflags are masked when a syscall instruction is executed -
// if a bit is set in sfmask, the corresponding bit in rflags is cleared. The value set here clears most of rflags,
// but keeps the reserved and virtualization bits intact. The userspace rflags value is saved in r11 by syscall.
constexpr u64 rflags_mask = 0x257fd5u;
MSR sfmask_msr(MSR_SFMASK);
sfmask_msr.set(rflags_mask);
#endif
}