The following commit has been merged into the x86/entry branch of tip:
Commit-ID: ca282b486a570a0bfda5c1a4595ace7fa14243bf
Gitweb: https://git.kernel.org/tip/ca282b486a570a0bfda5c1a4595ace7fa14243bf
Author: Brian Gerst <brgerst@xxxxxxxxx>
AuthorDate: Wed, 11 Oct 2023 18:43:49 -04:00
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Fri, 13 Oct 2023 13:05:28 +02:00
x86/entry/64: Convert SYSRET validation tests to C
No change in functionality expected.
Signed-off-by: Brian Gerst <brgerst@xxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Brian Gerst <brgerst@xxxxxxxxx>
Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Cc: Uros Bizjak <ubizjak@xxxxxxxxx>
Link: https://lore.kernel.org/r/20231011224351.130935-2-brgerst@xxxxxxxxx
---
arch/x86/entry/common.c | 43 ++++++++++++++++++++++++++-
arch/x86/entry/entry_64.S | 53 +--------------------------------
arch/x86/include/asm/syscall.h | 2 +-
3 files changed, 45 insertions(+), 53 deletions(-)
diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
index 0551bcb..9021465 100644
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -71,7 +71,8 @@ static __always_inline bool do_syscall_x32(struct pt_regs *regs, int nr)
return false;
}
-__visible noinstr void do_syscall_64(struct pt_regs *regs, int nr)
+/* Returns true to return using SYSRET, or false to use IRET */
+__visible noinstr bool do_syscall_64(struct pt_regs *regs, int nr)
{
add_random_kstack_offset();
nr = syscall_enter_from_user_mode(regs, nr);
@@ -85,6 +86,46 @@ __visible noinstr void do_syscall_64(struct pt_regs *regs, int nr)
instrumentation_end();
syscall_exit_to_user_mode(regs);
+
+ /*
+ * Check that the register state is valid for using SYSRET to exit
+ * to userspace. Otherwise use the slower but fully capable IRET
+ * exit path.
+ */
+
+ /* XEN PV guests always use the IRET path */
+ if (cpu_feature_enabled(X86_FEATURE_XENPV))
+ return false;
+
+ /* SYSRET requires RCX == RIP and R11 == EFLAGS */
+ if (unlikely(regs->cx != regs->ip || regs->r11 != regs->flags))
+ return false;
+
+ /* CS and SS must match the values set in MSR_STAR */
+ if (unlikely(regs->cs != __USER_CS || regs->ss != __USER_DS))
+ return false;
+
+ /*
+ * On Intel CPUs, SYSRET with non-canonical RCX/RIP will #GP
+ * in kernel space. This essentially lets the user take over
+ * the kernel, since userspace controls RSP.
+ *
+ * Change top bits to match the most significant bit (47th or 56th bit
+ * depending on paging mode) in the address.
+ */
+ if (unlikely(!__is_canonical_address(regs->ip, __VIRTUAL_MASK_SHIFT + 1)))
+ return false;
+
+ /*
+ * SYSRET cannot restore RF. It can restore TF, but unlike IRET,
+ * restoring TF results in a trap from userspace immediately after
+ * SYSRET.
+ */
+ if (unlikely(regs->flags & (X86_EFLAGS_RF | X86_EFLAGS_TF)))
+ return false;
+
+ /* Use SYSRET to exit to userspace */
+ return true;
}
#endif
diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 7574639..1730640 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -126,57 +126,8 @@ SYM_INNER_LABEL(entry_SYSCALL_64_after_hwframe, SYM_L_GLOBAL)
* In the Xen PV case we must use iret anyway.
*/
- ALTERNATIVE "", "jmp swapgs_restore_regs_and_return_to_usermode", \
- X86_FEATURE_XENPV
-
- movq RCX(%rsp), %rcx
- movq RIP(%rsp), %r11
-
- cmpq %rcx, %r11 /* SYSRET requires RCX == RIP */
- jne swapgs_restore_regs_and_return_to_usermode
-
- /*
- * On Intel CPUs, SYSRET with non-canonical RCX/RIP will #GP
- * in kernel space. This essentially lets the user take over
- * the kernel, since userspace controls RSP.
- *
- * If width of "canonical tail" ever becomes variable, this will need
- * to be updated to remain correct on both old and new CPUs.
- *
- * Change top bits to match most significant bit (47th or 56th bit
- * depending on paging mode) in the address.
- */
-#ifdef CONFIG_X86_5LEVEL
- ALTERNATIVE "shl $(64 - 48), %rcx; sar $(64 - 48), %rcx", \
- "shl $(64 - 57), %rcx; sar $(64 - 57), %rcx", X86_FEATURE_LA57
-#else
- shl $(64 - (__VIRTUAL_MASK_SHIFT+1)), %rcx
- sar $(64 - (__VIRTUAL_MASK_SHIFT+1)), %rcx
-#endif
-
- /* If this changed %rcx, it was not canonical */
- cmpq %rcx, %r11
- jne swapgs_restore_regs_and_return_to_usermode
-
- cmpq $__USER_CS, CS(%rsp) /* CS must match SYSRET */
- jne swapgs_restore_regs_and_return_to_usermode
-
- movq R11(%rsp), %r11
- cmpq %r11, EFLAGS(%rsp) /* R11 == RFLAGS */
- jne swapgs_restore_regs_and_return_to_usermode
-
- /*
- * SYSRET cannot restore RF. It can restore TF, but unlike IRET,
- * restoring TF results in a trap from userspace immediately after
- * SYSRET.
- */
- testq $(X86_EFLAGS_RF|X86_EFLAGS_TF), %r11
- jnz swapgs_restore_regs_and_return_to_usermode
-
- /* nothing to check for RSP */
-
- cmpq $__USER_DS, SS(%rsp) /* SS must match SYSRET */
- jne swapgs_restore_regs_and_return_to_usermode
+ ALTERNATIVE "testb %al, %al; jz swapgs_restore_regs_and_return_to_usermode", \
+ "jmp swapgs_restore_regs_and_return_to_usermode", X86_FEATURE_XENPV
/*
* We win! This label is here just for ease of understanding
diff --git a/arch/x86/include/asm/syscall.h b/arch/x86/include/asm/syscall.h
index c7e25c9..f44e2f9 100644
--- a/arch/x86/include/asm/syscall.h
+++ b/arch/x86/include/asm/syscall.h
@@ -126,7 +126,7 @@ static inline int syscall_get_arch(struct task_struct *task)
? AUDIT_ARCH_I386 : AUDIT_ARCH_X86_64;
}
-void do_syscall_64(struct pt_regs *regs, int nr);
+bool do_syscall_64(struct pt_regs *regs, int nr);
#endif /* CONFIG_X86_32 */
|