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Subject[PATCH] x86/asm/entry/64: Disentangle error_entry/exit gsbase/ebx/usermode code
Date
The error_entry/error_exit code to handle gsbase and whether we return
to user mdoe was a mess:

- error_sti was misnamed. In particular, it did not enable interrupts.

- Error handling for gs_change was hopelessly tangled the normal
usermode path. Separate it out. This saves a branch in normal
entries from kernel mode.

- The comments were bad.

Fix it up. As a nice side effect, there's now a code path that happens
on error entries from user mode. We'll use it soon.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
arch/x86/entry/entry_64.S | 39 +++++++++++++++++++++++++++++++--------
1 file changed, 31 insertions(+), 8 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index bd97161f90cb..3462a4f09425 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -1135,7 +1135,7 @@ END(paranoid_exit)

/*
* Save all registers in pt_regs, and switch gs if needed.
- * Return: ebx=0: need swapgs on exit, ebx=1: otherwise
+ * Return: ebx=0: came from user mode; ebx=1: otherwise
*/
ENTRY(error_entry)
cld
@@ -1144,9 +1144,11 @@ ENTRY(error_entry)
xorl %ebx, %ebx
testb $3, CS+8(%rsp)
jz error_kernelspace
-error_swapgs:
+
+ /* We entered from user mode */
SWAPGS
-error_sti:
+
+error_entry_done:
TRACE_IRQS_OFF
ret

@@ -1165,8 +1167,15 @@ error_kernelspace:
cmpq %rax, RIP+8(%rsp)
je bstep_iret
cmpq $gs_change, RIP+8(%rsp)
- je error_swapgs
- jmp error_sti
+ jne error_entry_done
+
+ /*
+ * hack: gs_change can fail with user gsbase. If this happens, fix up
+ * gsbase and proceed. We'll fix up the exception and land in
+ * gs_change's error handler with kernel gsbase.
+ */
+ SWAPGS
+ jmp error_entry_done

bstep_iret:
/* Fix truncated RIP */
@@ -1174,16 +1183,30 @@ bstep_iret:
/* fall through */

error_bad_iret:
+ /*
+ * We came from an IRET to user mode, so we have user gsbase.
+ * Switch to kernel gsbase.
+ */
SWAPGS
+
+ /*
+ * Pretend that the exception came from user mode: set up pt_regs
+ * as if we faulted immediately after IRET and zero ebx so that
+ * error_exit knows that we will returning to user mode.
+ */
mov %rsp, %rdi
call fixup_bad_iret
mov %rax, %rsp
- decl %ebx /* Return to usergs */
- jmp error_sti
+ decl %ebx
+ jmp error_entry_done
END(error_entry)


-/* On entry, ebx is "no swapgs" flag (1: don't need swapgs, 0: need it) */
+/*
+ * On entry, ebx is "return to kernel" flag
+ * 1: don't need swapgs
+ * 0: need swapgs and standard preparation for return to usermode
+ */
ENTRY(error_exit)
movl %ebx, %eax
RESTORE_EXTRA_REGS
--
2.4.2


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