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Peter Anvin" Subject: [PATCH V3 3/7] x86/entry: Switch the stack after error_entry() returns Date: Tue, 15 Mar 2022 15:39:45 +0800 Message-Id: <20220315073949.7541-4-jiangshanlai@gmail.com> X-Mailer: git-send-email 2.19.1.6.gb485710b In-Reply-To: <20220315073949.7541-1-jiangshanlai@gmail.com> References: <20220315073949.7541-1-jiangshanlai@gmail.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Lai Jiangshan error_entry() calls sync_regs() to settle/copy the pt_regs and switches the stack directly after sync_regs(). But error_entry() itself is also a function call, the switching has to handle the return address of it together, which causes the work complicated and tangly. Switching to the stack after error_entry() makes the code simpler and intuitive. The behavior/logic is unchanged: 1) (opt) feed fixup_bad_iret() with the pt_regs pushed by ASM code 2) (opt) fixup_bad_iret() moves the partial pt_regs up 3) feed sync_regs() with the pt_regs pushed by ASM code or returned by fixup_bad_iret() 4) sync_regs() copies the whole pt_regs to kernel stack if needed 5) after error_entry() and switching %rsp, it is in kernel stack with the pt_regs Changes only in calling: Old code switches to copied pt_regs immediately twice in error_entry() while new code switches to the copied pt_regs only once after error_entry() returns. It is correct since sync_regs() doesn't need to be called close to the pt_regs it handles. Old code stashes the return-address of error_entry() in a scratch register and new code doesn't stash it. It relies on the fact that fixup_bad_iret() and sync_regs() don't corrupt the return-address of error_entry() on the stack. But even the old code also relies on the fact that fixup_bad_iret() and sync_regs() don't corrupt the return-address of themselves. They are the same reliances and are assured. After this change, error_entry() will not do fancy things with the stack except when in the prolog which will be fixed in the next patch ("move PUSH_AND_CLEAR_REGS out of error_entry"). This patch and the next patch can't be swapped because the next patch relies on this patch's stopping fiddling with the return-address of error_entry(), otherwise the objtool would complain. Signed-off-by: Lai Jiangshan --- arch/x86/entry/entry_64.S | 16 ++++++---------- 1 file changed, 6 insertions(+), 10 deletions(-) diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S index 37505331b7f1..7768cdd0c7ed 100644 --- a/arch/x86/entry/entry_64.S +++ b/arch/x86/entry/entry_64.S @@ -326,6 +326,8 @@ SYM_CODE_END(ret_from_fork) .macro idtentry_body cfunc has_error_code:req call error_entry + movq %rax, %rsp /* switch stack settled by sync_regs() */ + ENCODE_FRAME_POINTER UNWIND_HINT_REGS movq %rsp, %rdi /* pt_regs pointer into 1st argument*/ @@ -1014,14 +1016,10 @@ SYM_CODE_START_LOCAL(error_entry) /* We have user CR3. Change to kernel CR3. */ SWITCH_TO_KERNEL_CR3 scratch_reg=%rax + leaq 8(%rsp), %rdi /* arg0 = pt_regs pointer */ .Lerror_entry_from_usermode_after_swapgs: /* Put us onto the real thread stack. */ - popq %r12 /* save return addr in %12 */ - movq %rsp, %rdi /* arg0 = pt_regs pointer */ call sync_regs - movq %rax, %rsp /* switch stack */ - ENCODE_FRAME_POINTER - pushq %r12 RET /* @@ -1053,6 +1051,7 @@ SYM_CODE_START_LOCAL(error_entry) */ .Lerror_entry_done_lfence: FENCE_SWAPGS_KERNEL_ENTRY + leaq 8(%rsp), %rax /* return pt_regs pointer */ RET .Lbstep_iret: @@ -1073,12 +1072,9 @@ SYM_CODE_START_LOCAL(error_entry) * Pretend that the exception came from user mode: set up pt_regs * as if we faulted immediately after IRET. */ - popq %r12 /* save return addr in %12 */ - movq %rsp, %rdi /* arg0 = pt_regs pointer */ + leaq 8(%rsp), %rdi /* arg0 = pt_regs pointer */ call fixup_bad_iret - mov %rax, %rsp - ENCODE_FRAME_POINTER - pushq %r12 + mov %rax, %rdi jmp .Lerror_entry_from_usermode_after_swapgs SYM_CODE_END(error_entry) -- 2.19.1.6.gb485710b