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Subject[PATCH V3 7/7] x86/entry: Implement the whole error_entry() as C code
Date
From: Lai Jiangshan <jiangshan.ljs@antgroup.com>

Implement error_entry() as C code in arch/x86/entry/entry64.c and
replace the ASM version of error_entry().

The code might be in the user CR3 and user GS base at the start of
the function so it calls __always_inline C function only until the GS
and CR3 is switched.

No functional change intended and comments are also copied.

The C version generally has better readability and easier to be
updated/improved.

Note:
To avoid using goto, the C code has two call sites of sync_regs().
It calls sync_regs() directly after fixup_bad_iret() returns while the
ASM code uses JMP instruction to jump to the start of the first call
site.

The complier uses tail-call-optimization for calling sync_regs(). It
uses "JMP sync_regs" while the ASM code uses "CALL+RET".

Signed-off-by: Lai Jiangshan <jiangshan.ljs@antgroup.com>
---
arch/x86/entry/entry64.c | 69 +++++++++++++++++++++++++++++
arch/x86/entry/entry_64.S | 85 ++----------------------------------
arch/x86/include/asm/proto.h | 1 +
arch/x86/include/asm/traps.h | 1 +
arch/x86/kernel/traps.c | 2 -
5 files changed, 74 insertions(+), 84 deletions(-)

diff --git a/arch/x86/entry/entry64.c b/arch/x86/entry/entry64.c
index f7f23800cee4..bd047c329622 100644
--- a/arch/x86/entry/entry64.c
+++ b/arch/x86/entry/entry64.c
@@ -13,6 +13,8 @@
*/
#include <asm/traps.h>

+extern unsigned char asm_load_gs_index_gs_change[];
+
#ifdef CONFIG_PAGE_TABLE_ISOLATION
static __always_inline void pti_switch_to_kernel_cr3(unsigned long user_cr3)
{
@@ -66,3 +68,70 @@ static __always_inline void user_entry_swapgs_and_fence(void)
native_swapgs();
fence_swapgs_user_entry();
}
+
+/*
+ * Put pt_regs onto the task stack and switch GS and CR3 if needed.
+ * The actual stack switch is done in entry_64.S.
+ *
+ * Be careful, it might be in the user CR3 and user GS base at the start
+ * of the function.
+ */
+asmlinkage __visible __entry_text
+struct pt_regs *error_entry(struct pt_regs *eregs)
+{
+ unsigned long iret_ip = (unsigned long)native_irq_return_iret;
+
+ if (user_mode(eregs)) {
+ /*
+ * We entered from user mode.
+ * Switch to kernel gsbase and CR3.
+ */
+ user_entry_swapgs_and_fence();
+ switch_to_kernel_cr3();
+
+ /* Put pt_regs onto the task stack. */
+ return sync_regs(eregs);
+ }
+
+ /*
+ * There are two places in the kernel that can potentially fault with
+ * usergs. Handle them here. B stepping K8s sometimes report a
+ * truncated RIP for IRET exceptions returning to compat mode. Check
+ * for these here too.
+ */
+ if ((eregs->ip == iret_ip) || (eregs->ip == (unsigned int)iret_ip)) {
+ eregs->ip = iret_ip; /* Fix truncated RIP */
+
+ /*
+ * We came from an IRET to user mode, so we have user
+ * gsbase and CR3. Switch to kernel gsbase and CR3:
+ */
+ user_entry_swapgs_and_fence();
+ switch_to_kernel_cr3();
+
+ /*
+ * Pretend that the exception came from user mode: set up
+ * pt_regs as if we faulted immediately after IRET and then
+ * put pt_regs onto the real task stack.
+ */
+ return sync_regs(fixup_bad_iret(eregs));
+ }
+
+ /*
+ * Hack: asm_load_gs_index_gs_change can fail with user gsbase.
+ * If this happens, fix up gsbase and proceed. We'll fix up the
+ * exception and land in asm_load_gs_index_gs_change's error
+ * handler with kernel gsbase.
+ */
+ if (eregs->ip == (unsigned long)asm_load_gs_index_gs_change)
+ native_swapgs();
+
+ /*
+ * Issue an LFENCE to prevent GS speculation, regardless of whether
+ * it is a kernel or user gsbase.
+ */
+ fence_swapgs_kernel_entry();
+
+ /* Enter from kernel, don't move pt_regs */
+ return eregs;
+}
diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 4300ba49b5ee..f8322398fe1c 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -348,7 +348,7 @@ SYM_CODE_END(push_and_clear_regs)
* own pvops for IRET and load_gs_index(). And it doesn't need to
* switch the CR3. So it can skip invoking error_entry().
*/
- ALTERNATIVE "call error_entry; movq %rax, %rsp", \
+ ALTERNATIVE "movq %rsp, %rdi; call error_entry; movq %rax, %rsp", \
"", X86_FEATURE_XENPV

ENCODE_FRAME_POINTER
@@ -784,7 +784,7 @@ _ASM_NOKPROBE(common_interrupt_return)
SYM_FUNC_START(asm_load_gs_index)
FRAME_BEGIN
swapgs
-.Lgs_change:
+SYM_INNER_LABEL(asm_load_gs_index_gs_change, SYM_L_GLOBAL)
ANNOTATE_NOENDBR // error_entry
movl %edi, %gs
2: ALTERNATIVE "", "mfence", X86_BUG_SWAPGS_FENCE
@@ -805,7 +805,7 @@ SYM_FUNC_START(asm_load_gs_index)
movl %eax, %gs
jmp 2b

- _ASM_EXTABLE(.Lgs_change, .Lbad_gs)
+ _ASM_EXTABLE(asm_load_gs_index_gs_change, .Lbad_gs)

SYM_FUNC_END(asm_load_gs_index)
EXPORT_SYMBOL(asm_load_gs_index)
@@ -1012,85 +1012,6 @@ SYM_CODE_START_LOCAL(paranoid_exit)
jmp restore_regs_and_return_to_kernel
SYM_CODE_END(paranoid_exit)

-/*
- * Switch GS and CR3 if needed.
- */
-SYM_CODE_START_LOCAL(error_entry)
- UNWIND_HINT_FUNC
- testb $3, CS+8(%rsp)
- jz .Lerror_kernelspace
-
- /*
- * We entered from user mode or we're pretending to have entered
- * from user mode due to an IRET fault.
- */
- swapgs
- FENCE_SWAPGS_USER_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. */
- call sync_regs
- RET
-
- /*
- * There are two places in the kernel that can potentially fault with
- * usergs. Handle them here. B stepping K8s sometimes report a
- * truncated RIP for IRET exceptions returning to compat mode. Check
- * for these here too.
- */
-.Lerror_kernelspace:
- leaq native_irq_return_iret(%rip), %rcx
- cmpq %rcx, RIP+8(%rsp)
- je .Lerror_bad_iret
- movl %ecx, %eax /* zero extend */
- cmpq %rax, RIP+8(%rsp)
- je .Lbstep_iret
- cmpq $.Lgs_change, RIP+8(%rsp)
- jne .Lerror_entry_done_lfence
-
- /*
- * hack: .Lgs_change can fail with user gsbase. If this happens, fix up
- * gsbase and proceed. We'll fix up the exception and land in
- * .Lgs_change's error handler with kernel gsbase.
- */
- swapgs
-
- /*
- * Issue an LFENCE to prevent GS speculation, regardless of whether it is a
- * kernel or user gsbase.
- */
-.Lerror_entry_done_lfence:
- FENCE_SWAPGS_KERNEL_ENTRY
- leaq 8(%rsp), %rax /* return pt_regs pointer */
- RET
-
-.Lbstep_iret:
- /* Fix truncated RIP */
- movq %rcx, RIP+8(%rsp)
- /* fall through */
-
-.Lerror_bad_iret:
- /*
- * We came from an IRET to user mode, so we have user
- * gsbase and CR3. Switch to kernel gsbase and CR3:
- */
- swapgs
- FENCE_SWAPGS_USER_ENTRY
- SWITCH_TO_KERNEL_CR3 scratch_reg=%rax
-
- /*
- * Pretend that the exception came from user mode: set up pt_regs
- * as if we faulted immediately after IRET.
- */
- leaq 8(%rsp), %rdi /* arg0 = pt_regs pointer */
- call fixup_bad_iret
- mov %rax, %rdi
- jmp .Lerror_entry_from_usermode_after_swapgs
-SYM_CODE_END(error_entry)
-
SYM_CODE_START_LOCAL(error_return)
UNWIND_HINT_REGS
DEBUG_ENTRY_ASSERT_IRQS_OFF
diff --git a/arch/x86/include/asm/proto.h b/arch/x86/include/asm/proto.h
index 12ef86b19910..199d27fbf903 100644
--- a/arch/x86/include/asm/proto.h
+++ b/arch/x86/include/asm/proto.h
@@ -15,6 +15,7 @@ void entry_SYSCALL_64(void);
void entry_SYSCALL_64_safe_stack(void);
void entry_SYSRETQ_unsafe_stack(void);
void entry_SYSRETQ_end(void);
+extern unsigned char native_irq_return_iret[];
long do_arch_prctl_64(struct task_struct *task, int option, unsigned long arg2);
#endif

diff --git a/arch/x86/include/asm/traps.h b/arch/x86/include/asm/traps.h
index 47ecfff2c83d..2d00100d3e03 100644
--- a/arch/x86/include/asm/traps.h
+++ b/arch/x86/include/asm/traps.h
@@ -14,6 +14,7 @@
asmlinkage __visible notrace struct pt_regs *sync_regs(struct pt_regs *eregs);
asmlinkage __visible notrace
struct pt_regs *fixup_bad_iret(struct pt_regs *bad_regs);
+asmlinkage __visible notrace struct pt_regs *error_entry(struct pt_regs *eregs);
void __init trap_init(void);
asmlinkage __visible noinstr struct pt_regs *vc_switch_off_ist(struct pt_regs *eregs);
#endif
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index d62b2cb85cea..f76a15f654c5 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -436,8 +436,6 @@ DEFINE_IDTENTRY_DF(exc_double_fault)
#endif

#ifdef CONFIG_X86_ESPFIX64
- extern unsigned char native_irq_return_iret[];
-
/*
* If IRET takes a non-IST fault on the espfix64 stack, then we
* end up promoting it to a doublefault. In that case, take
--
2.19.1.6.gb485710b
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