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Date: Sat, 18 Jul 2020 10:38:37 -0400
From: Brian Gerst <brgerst@...il.com>
To: linux-kernel@...r.kernel.org, x86@...nel.org
Cc: Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
"H . Peter Anvin" <hpa@...or.com>,
Andy Lutomirski <luto@...nel.org>,
Brian Gerst <brgerst@...il.com>
Subject: [PATCH 1/3] x86-64: Move SYSRET validation code to C
Signed-off-by: Brian Gerst <brgerst@...il.com>
---
arch/x86/entry/calling.h | 10 +----
arch/x86/entry/common.c | 56 ++++++++++++++++++++++++++-
arch/x86/entry/entry_64.S | 71 ++--------------------------------
arch/x86/include/asm/syscall.h | 2 +-
4 files changed, 60 insertions(+), 79 deletions(-)
diff --git a/arch/x86/entry/calling.h b/arch/x86/entry/calling.h
index 98e4d8886f11c..904477d3e388f 100644
--- a/arch/x86/entry/calling.h
+++ b/arch/x86/entry/calling.h
@@ -147,27 +147,19 @@ For 32-bit we have the following conventions - kernel is built with
.endm
-.macro POP_REGS pop_rdi=1 skip_r11rcx=0
+.macro POP_REGS pop_rdi=1
popq %r15
popq %r14
popq %r13
popq %r12
popq %rbp
popq %rbx
- .if \skip_r11rcx
- popq %rsi
- .else
popq %r11
- .endif
popq %r10
popq %r9
popq %r8
popq %rax
- .if \skip_r11rcx
- popq %rsi
- .else
popq %rcx
- .endif
popq %rdx
popq %rsi
.if \pop_rdi
diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
index 54ad1890aefca..9e01445f6679c 100644
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -365,9 +365,11 @@ __visible noinstr void syscall_return_slowpath(struct pt_regs *regs)
}
#ifdef CONFIG_X86_64
-__visible noinstr void do_syscall_64(unsigned long nr, struct pt_regs *regs)
+__visible noinstr bool do_syscall_64(unsigned long nr, struct pt_regs *regs)
{
struct thread_info *ti;
+ long rip;
+ unsigned int shift_rip;
check_user_regs(regs);
@@ -394,6 +396,58 @@ __visible noinstr void do_syscall_64(unsigned long nr, struct pt_regs *regs)
instrumentation_end();
exit_to_user_mode();
+
+ /*
+ * Check that the register state is valid for using SYSRET to exit
+ * to userspace. Otherwise use the slower IRET exit path.
+ */
+
+ /* SYSRET requires RCX == RIP and R11 = EFLAGS */
+ if (unlikely(regs->cx != regs->ip || regs->r11 != regs->flags))
+ return false;
+
+ /* CS and SS must match 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 most significant bit (47th or 56th bit
+ * depending on paging mode) in the address.
+ */
+ shift_rip = (64 - __VIRTUAL_MASK_SHIFT + 1);
+ rip = (long) regs->ip;
+ rip <<= shift_rip;
+ rip >>= shift_rip;
+ if (unlikely((unsigned long) rip != regs->ip))
+ return false;
+
+ /*
+ * SYSCALL clears RF when it saves RFLAGS in R11 and SYSRET cannot
+ * restore RF properly. If the slowpath sets it for whatever reason, we
+ * need to restore it correctly.
+ *
+ * SYSRET can restore TF, but unlike IRET, restoring TF results in a
+ * trap from userspace immediately after SYSRET. This would cause an
+ * infinite loop whenever #DB happens with register state that satisfies
+ * the opportunistic SYSRET conditions. For example, single-stepping
+ * this user code:
+ *
+ * movq $stuck_here, %rcx
+ * pushfq
+ * popq %r11
+ * stuck_here:
+ *
+ * would never get past 'stuck_here'.
+ */
+ 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 fb729f4c4fbc2..b8025a62ac5e8 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -117,80 +117,15 @@ SYM_INNER_LABEL(entry_SYSCALL_64_after_hwframe, SYM_L_GLOBAL)
movq %rsp, %rsi
call do_syscall_64 /* returns with IRQs disabled */
- /*
- * Try to use SYSRET instead of IRET if we're returning to
- * a completely clean 64-bit userspace context. If we're not,
- * go to the slow exit path.
- */
- 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
-
- /*
- * SYSCALL clears RF when it saves RFLAGS in R11 and SYSRET cannot
- * restore RF properly. If the slowpath sets it for whatever reason, we
- * need to restore it correctly.
- *
- * SYSRET can restore TF, but unlike IRET, restoring TF results in a
- * trap from userspace immediately after SYSRET. This would cause an
- * infinite loop whenever #DB happens with register state that satisfies
- * the opportunistic SYSRET conditions. For example, single-stepping
- * this user code:
- *
- * movq $stuck_here, %rcx
- * pushfq
- * popq %r11
- * stuck_here:
- *
- * would never get past 'stuck_here'.
- */
- 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
+ testb %al, %al /* Is SYSRET allowed? */
+ jz swapgs_restore_regs_and_return_to_usermode
/*
* We win! This label is here just for ease of understanding
* perf profiles. Nothing jumps here.
*/
syscall_return_via_sysret:
- /* rcx and r11 are already restored (see code above) */
- POP_REGS pop_rdi=0 skip_r11rcx=1
+ POP_REGS pop_rdi=0
/*
* Now all regs are restored except RSP and RDI.
diff --git a/arch/x86/include/asm/syscall.h b/arch/x86/include/asm/syscall.h
index 7cbf733d11afd..766f9b9736185 100644
--- a/arch/x86/include/asm/syscall.h
+++ b/arch/x86/include/asm/syscall.h
@@ -160,7 +160,7 @@ static inline int syscall_get_arch(struct task_struct *task)
? AUDIT_ARCH_I386 : AUDIT_ARCH_X86_64;
}
-void do_syscall_64(unsigned long nr, struct pt_regs *regs);
+bool do_syscall_64(unsigned long nr, struct pt_regs *regs);
void do_int80_syscall_32(struct pt_regs *regs);
long do_fast_syscall_32(struct pt_regs *regs);
--
2.26.2
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