| From | Andy Lutomirski <> | Date | Wed, 14 Oct 2015 09:25:24 -0700 | Subject | Re: [PATCH v2 24/36] x86/entry/compat: Implement opportunistic SYSRETL for compat syscalls |
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On Mon, Oct 12, 2015 at 9:15 AM, Borislav Petkov <bp@alien8.de> wrote: > On Mon, Oct 05, 2015 at 05:48:12PM -0700, Andy Lutomirski wrote: >> If CS, SS and IP are as expected and FLAGS is compatible with SYSRETL, >> then return from fast compat syscalls (both SYSCALL and SYSENTER) using >> SYSRETL. >> >> Unlike native 64-bit opportunistic SYSRET, this is not invisible to >> user code: RCX and R8-R15 end up in a different state than shown >> saved in pt_regs. To compensate, we only do this when returning to >> the vDSO fast syscall return path. This won't interfere with >> syscall restart, as we won't use SYSRETL when returning to the INT80 >> restart instruction. >> >> Signed-off-by: Andy Lutomirski <luto@kernel.org> >> --- >> arch/x86/entry/common.c | 23 +++++++++++++++++++--- >> arch/x86/entry/entry_64_compat.S | 42 ++++++++++++++++++++++++++++++++++++++-- >> 2 files changed, 60 insertions(+), 5 deletions(-) >> >> diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c >> index 1b2606edc621..88dc5ba14d47 100644 >> --- a/arch/x86/entry/common.c >> +++ b/arch/x86/entry/common.c >> @@ -363,7 +363,8 @@ __visible void do_int80_syscall_32(struct pt_regs *regs) >> syscall_return_slowpath(regs); >> } >> >> -__visible void do_fast_syscall_32(struct pt_regs *regs) >> +/* Returns 0 to return using IRET or 1 to return using SYSRETL. */ >> +__visible long do_fast_syscall_32(struct pt_regs *regs) >> { >> /* >> * Called using the internal vDSO SYSENTER/SYSCALL32 calling >> @@ -395,12 +396,28 @@ __visible void do_fast_syscall_32(struct pt_regs *regs) >> enter_from_user_mode(); >> #endif >> prepare_exit_to_usermode(regs); >> - return; >> + return 0; /* Keep it simple: use IRET. */ >> } >> local_irq_disable(); >> >> /* Now this is just like a normal syscall. */ >> do_int80_syscall_32(regs); >> - return; >> + >> +#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. >> + */ >> + return regs->cs == __USER32_CS && regs->ss == __USER_DS && >> + regs->ip == landing_pad && >> + (regs->flags & (X86_EFLAGS_RF | X86_EFLAGS_TF)) == 0; > > This could've used some readability massaging: > > return regs->cs == __USER32_CS && > regs->ss == __USER_DS && > regs->ip == landing_pad && > !(regs->flags & (X86_EFLAGS_RF | X86_EFLAGS_TF)); > > Although I'm not crazy about that one either. This expression is simply not > parseable at a glance. > > Why not write it like the 64-bit opportunistic SYSRET: > > if (regs->cs != __USER32_CS) > return 0; > > if (regs->ss != __USER_DS) > return 0; > > and so on...
We could do that. It might also be nice to factor it into its own function:
static bool may_fast_exit_32(struct pt_regs *regs);
I'll try that eventually if you don't beat me.
--Andy
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