[tip: x86/fred] x86/fred: Fixup fault on ERETU by jumping to fred_entrypoint_user

tip-bot2 for Xin Li
2024-01-31T21:14:45Z

Commit-ID: 5105e7687ad3dffde77f6e4393b5530e83d672dc
Gitweb: https://git.kernel.org/tip/5105e7687ad3dffde77f6e4393b5530e83d672dc
Author: Xin Li <xin3.li@intel.com>
AuthorDate: Tue, 05 Dec 2023 02:50:18 -08:00
Committer: Borislav Petkov (AMD) <bp@alien8.de>
CommitterDate: Wed, 31 Jan 2024 22:03:04 +01:00

x86/fred: Fixup fault on ERETU by jumping to fred_entrypoint_user

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>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Tested-by: Shan Kang <shan.kang@intel.com>
Link: https://lore.kernel.org/r/20231205105030.8698-30-xin3.li@intel.com
---
arch/x86/entry/entry_64_fred.S | 5 +-
arch/x86/include/asm/extable_fixup_types.h | 4 +-
arch/x86/mm/extable.c | 78 +++++++++++++++++++++-
3 files changed, 85 insertions(+), 2 deletions(-)

diff --git a/arch/x86/entry/entry_64_fred.S b/arch/x86/entry/entry_64_fred.S
index 2271a1c..7fe2722 100644
--- a/arch/x86/entry/entry_64_fred.S
+++ b/arch/x86/entry/entry_64_fred.S
@@ -3,6 +3,7 @@
* The actual FRED entry points.
*/

+#include <asm/asm.h>
#include <asm/fred.h>

#include "calling.h"
@@ -34,7 +35,9 @@ SYM_CODE_START_NOALIGN(asm_fred_entrypoint_user)
call fred_entry_from_user
SYM_INNER_LABEL(asm_fred_exit_user, SYM_L_GLOBAL)
FRED_EXIT
- ERETU
+1: ERETU
+
+ _ASM_EXTABLE_TYPE(1b, asm_fred_entrypoint_user, EX_TYPE_ERETU)
SYM_CODE_END(asm_fred_entrypoint_user)

/*
diff --git a/arch/x86/include/asm/extable_fixup_types.h b/arch/x86/include/asm/extable_fixup_types.h
index fe63120..7acf038 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 271dcb2..b522933 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>
@@ -223,6 +224,79 @@ static bool ex_handler_ucopy_len(const struct exception_table_entry *fixup,
return ex_handler_uaccess(fixup, regs, trapnr, fault_address);
}

+#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, orig_ax));
+ unsigned short ss = uregs->ss;
+ unsigned short cs = uregs->cs;
+
+ /*
+ * Move the NMI bit from the invalid stack frame, which caused ERETU
+ * to fault, to the fault handler's stack frame, thus to unblock NMI
+ * with the fault handler's ERETS instruction ASAP if NMI is blocked.
+ */
+ regs->fred_ss.nmi = uregs->fred_ss.nmi;
+
+ /*
+ * Sync event information to uregs, i.e., the ERETU return frame, but
+ * is it safe to write to the ERETU return frame which is just above
+ * current event stack frame?
+ *
+ * The RSP used by FRED to push a stack frame is not the value in %rsp,
+ * it is calculated from %rsp with the following 2 steps:
+ * 1) RSP = %rsp - (IA32_FRED_CONFIG & 0x1c0) // Reserve N*64 bytes
+ * 2) RSP = RSP & ~0x3f // Align to a 64-byte cache line
+ * when an event delivery doesn't trigger a stack level change.
+ *
+ * Here is an example with N*64 (N=1) bytes reserved:
+ *
+ * 64-byte cache line ==> ______________
+ * |___Reserved___|
+ * |__Event_data__|
+ * |_____SS_______|
+ * |_____RSP______|
+ * |_____FLAGS____|
+ * |_____CS_______|
+ * |_____IP_______|
+ * 64-byte cache line ==> |__Error_code__| <== ERETU return frame
+ * |______________|
+ * |______________|
+ * |______________|
+ * |______________|
+ * |______________|
+ * |______________|
+ * |______________|
+ * 64-byte cache line ==> |______________| <== RSP after step 1) and 2)
+ * |___Reserved___|
+ * |__Event_data__|
+ * |_____SS_______|
+ * |_____RSP______|
+ * |_____FLAGS____|
+ * |_____CS_______|
+ * |_____IP_______|
+ * 64-byte cache line ==> |__Error_code__| <== ERETS return frame
+ *
+ * Thus a new FRED stack frame will always be pushed below a previous
+ * FRED stack frame ((N*64) bytes may be reserved between), and it is
+ * safe to write to a previous FRED stack frame as they never overlap.
+ */
+ fred_info(uregs)->edata = fred_event_data(regs);
+ uregs->ssx = regs->ssx;
+ uregs->fred_ss.ss = ss;
+ /* The NMI bit was moved away above */
+ uregs->fred_ss.nmi = 0;
+ uregs->csx = regs->csx;
+ uregs->fred_cs.sl = 0;
+ uregs->fred_cs.wfe = 0;
+ uregs->cs = cs;
+ uregs->orig_ax = error_code;
+
+ 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);
@@ -300,6 +374,10 @@ int fixup_exception(struct pt_regs *regs, int trapnr, unsigned long error_code,
return ex_handler_ucopy_len(e, regs, trapnr, fault_addr, 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();
}