x86/entry/compat: Implement opportunistic SYSRETL for compat syscalls

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>
Cc: Andy Lutomirski <luto@amacapital.net>
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: linux-kernel@vger.kernel.org
Link: http://lkml.kernel.org/r/aa15e49db33773eb10b73d73466b6d5466d7856a.1444091585.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
index 1b2606edc6212c326bdcede912bc3f599e1ba4dd..88dc5ba14d476f00a537a015ad2d3002d61bbbdb 100644 (file)
--- 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;
+#else
+       return 0;
+#endif
 }
 #endif

diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S
index 8f109de51d03474f3d4e425fe82b1e1b780a51dd..cf9641cd47965bd3c7ee7b04b142914da25cb69c 100644 (file)
--- a/arch/x86/entry/entry_64_compat.S
+++ b/arch/x86/entry/entry_64_compat.S
@@ -115,7 +115,9 @@ sysenter_flags_fixed:
 
        movq    %rsp, %rdi
        call    do_fast_syscall_32
-       jmp     .Lsyscall_32_done
+       testl   %eax, %eax
+       jz      .Lsyscall_32_done
+       jmp     sysret32_from_system_call
 
 sysenter_fix_flags:
        pushq   $X86_EFLAGS_FIXED
@@ -192,7 +194,43 @@ ENTRY(entry_SYSCALL_compat)
 
        movq    %rsp, %rdi
        call    do_fast_syscall_32
-       jmp     .Lsyscall_32_done
+       testl   %eax, %eax
+       jz      .Lsyscall_32_done
+
+       /* Opportunistic SYSRET */
+sysret32_from_system_call:
+       TRACE_IRQS_ON                   /* User mode traces as IRQs on. */
+       movq    RBX(%rsp), %rbx         /* pt_regs->rbx */
+       movq    RBP(%rsp), %rbp         /* pt_regs->rbp */
+       movq    EFLAGS(%rsp), %r11      /* pt_regs->flags (in r11) */
+       movq    RIP(%rsp), %rcx         /* pt_regs->ip (in rcx) */
+       addq    $RAX, %rsp              /* Skip r8-r15 */
+       popq    %rax                    /* pt_regs->rax */
+       popq    %rdx                    /* Skip pt_regs->cx */
+       popq    %rdx                    /* pt_regs->dx */
+       popq    %rsi                    /* pt_regs->si */
+       popq    %rdi                    /* pt_regs->di */
+
+        /*
+         * USERGS_SYSRET32 does:
+         *  GSBASE = user's GS base
+         *  EIP = ECX
+         *  RFLAGS = R11
+         *  CS = __USER32_CS
+         *  SS = __USER_DS
+         *
+        * ECX will not match pt_regs->cx, but we're returning to a vDSO
+        * trampoline that will fix up RCX, so this is okay.
+        *
+        * R12-R15 are callee-saved, so they contain whatever was in them
+        * when the system call started, which is already known to user
+        * code.  We zero R8-R10 to avoid info leaks.
+         */
+       xorq    %r8, %r8
+       xorq    %r9, %r9
+       xorq    %r10, %r10
+       movq    RSP-ORIG_RAX(%rsp), %rsp
+        USERGS_SYSRET32
 END(entry_SYSCALL_compat)
 
 /*