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x86/entry/32: Clean up syscall fast exit tests

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Merge compat and native code and clarify comments.

No change in functionality expected.

Signed-off-by: Brian Gerst <brgerst@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Uros Bizjak <ubizjak@gmail.com>
Link: https://lore.kernel.org/r/20231011224351.130935-4-brgerst@gmail.com
This commit is contained in:
Brian Gerst 2023-10-11 18:43:51 -04:00 committed by Ingo Molnar
parent 58978b44df
commit 1a09a27153
1 changed files with 22 additions and 26 deletions
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48
arch/x86/entry/common.c
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@ -255,34 +255,30 @@ __visible noinstr bool do_fast_syscall_32(struct pt_regs *regs)
if (!__do_fast_syscall_32(regs))
return false;
#ifdef CONFIG_X86_64
/*
* Opportunistic SYSRETL: if possible, try to return using SYSRETL.
* SYSRETL is available on all 64-bit CPUs, so we don't need to
* bother with SYSEXIT.
*
* Unlike 64-bit opportunistic SYSRET, we can't check that CX == IP,
* because the ECX fixup above will ensure that this is essentially
* never the case.
* Check that the register state is valid for using SYSRETL/SYSEXIT
* to exit to userspace. Otherwise use the slower but fully capable
* IRET exit path.
*/
return regs->cs == __USER32_CS && regs->ss == __USER_DS &&
regs->ip == landing_pad &&
(regs->flags & (X86_EFLAGS_RF | X86_EFLAGS_TF)) == 0;
#else
/*
* Opportunistic SYSEXIT: if possible, try to return using SYSEXIT.
*
* Unlike 64-bit opportunistic SYSRET, we can't check that CX == IP,
* because the ECX fixup above will ensure that this is essentially
* never the case.
*
* We don't allow syscalls at all from VM86 mode, but we still
* need to check VM, because we might be returning from sys_vm86.
*/
return regs->cs == __USER_CS && regs->ss == __USER_DS &&
regs->ip == landing_pad &&
(regs->flags & (X86_EFLAGS_RF | X86_EFLAGS_TF | X86_EFLAGS_VM)) == 0;
#endif
/* XEN PV guests always use the IRET path */
if (cpu_feature_enabled(X86_FEATURE_XENPV))
return false;
/* EIP must point to the VDSO landing pad */
if (unlikely(regs->ip != landing_pad))
return false;
/* CS and SS must match the values set in MSR_STAR */
if (unlikely(regs->cs != __USER32_CS || regs->ss != __USER_DS))
return false;
/* If the TF, RF, or VM flags are set, use IRET */
if (unlikely(regs->flags & (X86_EFLAGS_RF | X86_EFLAGS_TF | X86_EFLAGS_VM)))
return false;
/* Use SYSRETL/SYSEXIT to exit to userspace */
return true;
}
/* Returns true to return using SYSEXIT/SYSRETL, or false to use IRET */