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x86/entry: Harden return-to-user

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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
This commit is contained in:
Peter Zijlstra 2023-11-20 15:33:46 +01:00 committed by Ingo Molnar
parent c516213726
commit 1e4d3001f5
1 changed files with 8 additions and 10 deletions
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18
arch/x86/entry/entry_64.S
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@ -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)