x86/entry: Harden return-to-user

Make the CONFIG_DEBUG_ENTRY=y check that validates CS is a user segment
unconditional and move it nearer to IRET.

  PRE:
       140,026,608      cycles:k                                                      ( +-  0.01% )
       236,696,176      instructions:k            #    1.69  insn per cycle           ( +-  0.00% )

  POST:
       139,957,681      cycles:k                                                      ( +-  0.01% )
       236,681,819      instructions:k            #    1.69  insn per cycle           ( +-  0.00% )

(this is with --repeat 100 and the run-to-run variance is bigger than
the difference shown)

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20231120143626.753200755@infradead.org

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index dfbf799b7311db7f9cd52fa2ad268b748d965a04..c40f89ab1b4c70a18b632a50c1e659e3fd83cfa9 100644 (file)
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -559,13 +559,6 @@ __irqentry_text_end:
 SYM_CODE_START_LOCAL(common_interrupt_return)
 SYM_INNER_LABEL(swapgs_restore_regs_and_return_to_usermode, SYM_L_GLOBAL)
        IBRS_EXIT
-#ifdef CONFIG_DEBUG_ENTRY
-       /* Assert that pt_regs indicates user mode. */
-       testb   $3, CS(%rsp)
-       jnz     1f
-       ud2
-1:
-#endif
 #ifdef CONFIG_XEN_PV
        ALTERNATIVE "", "jmp xenpv_restore_regs_and_return_to_usermode", X86_FEATURE_XENPV
 #endif
@@ -576,8 +569,14 @@ SYM_INNER_LABEL(swapgs_restore_regs_and_return_to_usermode, SYM_L_GLOBAL)
        STACKLEAK_ERASE
        POP_REGS
        add     $8, %rsp        /* orig_ax */
+       UNWIND_HINT_IRET_REGS
+
+.Lswapgs_and_iret:
        swapgs
-       jmp     .Lnative_iret
+       /* Assert that the IRET frame indicates user mode. */
+       testb   $3, 8(%rsp)
+       jnz     .Lnative_iret
+       ud2
 
 #ifdef CONFIG_PAGE_TABLE_ISOLATION
 .Lpti_restore_regs_and_return_to_usermode:
@@ -613,8 +612,7 @@ SYM_INNER_LABEL(swapgs_restore_regs_and_return_to_usermode, SYM_L_GLOBAL)
 
        /* Restore RDI. */
        popq    %rdi
-       swapgs
-       jmp     .Lnative_iret
+       jmp     .Lswapgs_and_iret
 #endif
 
 SYM_INNER_LABEL(restore_regs_and_return_to_kernel, SYM_L_GLOBAL)