[RFC PATCH v3 28/32] x86/fred: fixup fault on ERETU by jumping to fred_entrypoint_user

Xin Li
2023-02-24T07:29:56Z

If the stack frame contains an invalid user context (e.g. due to invalid SS,
a non-canonical RIP, etc.) the ERETU instruction will trap (#SS or #GP).

From a Linux point of view, this really should be considered a user space
failure, so use the standard fault fixup mechanism to intercept the fault,
fix up the exception frame, and redirect execution to fred_entrypoint_user.
The end result is that it appears just as if the hardware had taken the
exception immediately after completing the transition to user space.

Suggested-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Signed-off-by: Xin Li <xin3.li@intel.com>
---
arch/x86/entry/entry_64_fred.S | 8 +++++--
arch/x86/include/asm/extable_fixup_types.h | 4 +++-
arch/x86/mm/extable.c | 28 ++++++++++++++++++++++
3 files changed, 37 insertions(+), 3 deletions(-)

diff --git a/arch/x86/entry/entry_64_fred.S b/arch/x86/entry/entry_64_fred.S
index 1fb765fd3871..027ef8f1e600 100644
--- a/arch/x86/entry/entry_64_fred.S
+++ b/arch/x86/entry/entry_64_fred.S
@@ -5,8 +5,10 @@
* The actual FRED entry points.
*/
#include <linux/linkage.h>
-#include <asm/errno.h>
+#include <asm/asm.h>
#include <asm/asm-offsets.h>
+#include <asm/errno.h>
+#include <asm/export.h>
#include <asm/fred.h>

#include "calling.h"
@@ -38,7 +40,9 @@ SYM_CODE_START_NOALIGN(fred_entrypoint_user)
call fred_entry_from_user
SYM_INNER_LABEL(fred_exit_user, SYM_L_GLOBAL)
FRED_EXIT
- ERETU
+1: ERETU
+
+ _ASM_EXTABLE_TYPE(1b, fred_entrypoint_user, EX_TYPE_ERETU)
SYM_CODE_END(fred_entrypoint_user)

/*
diff --git a/arch/x86/include/asm/extable_fixup_types.h b/arch/x86/include/asm/extable_fixup_types.h
index 991e31cfde94..1585c798a02f 100644
--- a/arch/x86/include/asm/extable_fixup_types.h
+++ b/arch/x86/include/asm/extable_fixup_types.h
@@ -64,6 +64,8 @@
#define EX_TYPE_UCOPY_LEN4 (EX_TYPE_UCOPY_LEN | EX_DATA_IMM(4))
#define EX_TYPE_UCOPY_LEN8 (EX_TYPE_UCOPY_LEN | EX_DATA_IMM(8))

-#define EX_TYPE_ZEROPAD 20 /* longword load with zeropad on fault */
+#define EX_TYPE_ZEROPAD 20 /* longword load with zeropad on fault */
+
+#define EX_TYPE_ERETU 21

#endif
diff --git a/arch/x86/mm/extable.c b/arch/x86/mm/extable.c
index 60814e110a54..31542a2507ae 100644
--- a/arch/x86/mm/extable.c
+++ b/arch/x86/mm/extable.c
@@ -6,6 +6,7 @@
#include <xen/xen.h>

#include <asm/fpu/api.h>
+#include <asm/fred.h>
#include <asm/sev.h>
#include <asm/traps.h>
#include <asm/kdebug.h>
@@ -195,6 +196,29 @@ static bool ex_handler_ucopy_len(const struct exception_table_entry *fixup,
return ex_handler_uaccess(fixup, regs, trapnr);
}

+#ifdef CONFIG_X86_FRED
+static bool ex_handler_eretu(const struct exception_table_entry *fixup,
+ struct pt_regs *regs, unsigned long error_code)
+{
+ struct pt_regs *uregs = (struct pt_regs *)(regs->sp - offsetof(struct pt_regs, ip));
+ unsigned short ss = uregs->ss;
+ unsigned short cs = uregs->cs;
+
+ fred_info(uregs)->edata = fred_event_data(regs);
+ uregs->ssl = regs->ssl;
+ uregs->ss = ss;
+ uregs->csl = regs->csl;
+ uregs->current_stack_level = 0;
+ uregs->cs = cs;
+
+ /* Copy error code to uregs and adjust stack pointer accordingly */
+ uregs->orig_ax = error_code;
+ regs->sp -= 8;
+
+ return ex_handler_default(fixup, regs);
+}
+#endif
+
int ex_get_fixup_type(unsigned long ip)
{
const struct exception_table_entry *e = search_exception_tables(ip);
@@ -272,6 +296,10 @@ int fixup_exception(struct pt_regs *regs, int trapnr, unsigned long error_code,
return ex_handler_ucopy_len(e, regs, trapnr, reg, imm);
case EX_TYPE_ZEROPAD:
return ex_handler_zeropad(e, regs, fault_addr);
+#ifdef CONFIG_X86_FRED
+ case EX_TYPE_ERETU:
+ return ex_handler_eretu(e, regs, error_code);
+#endif
}
BUG();
}
--
2.34.1