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Subject[PATCH 2/2] x86/entry: Harden return-to-user
Make the DEBUG_ENTRY 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>
---
arch/x86/entry/entry_64.S | 18 ++++++++----------
1 file changed, 8 insertions(+), 10 deletions(-)

--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -559,13 +559,6 @@ SYM_CODE_END(\asmsym)
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_
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_

/* 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)

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