x86/paravirt: Fix spectre-v2 mitigations for paravirt guests

Peter Zijlstra · KAGA-KOKO · commit 5800dc5c19f3 · 2018-08-07T22:15:02.000+02:00
Nadav reported that on guests we're failing to rewrite the indirect calls to CALLEE_SAVE paravirt functions. In particular the pv_queued_spin_unlock() call is left unpatched and that is all over the place. This obviously wrecks Spectre-v2 mitigation (for paravirt guests) which relies on not actually having indirect calls around. The reason is an incorrect clobber test in paravirt_patch_call(); this function rewrites an indirect call with a direct call to the _SAME_ function, there is no possible way the clobbers can be different because of this. Therefore remove this clobber check. Also put WARNs on the other patch failure case (not enough room for the instruction) which I've not seen trigger in my (limited) testing. Three live kernel image disassemblies for lock_sock_nested (as a small function that illustrates the problem nicely). PRE is the current situation for guests, POST is with this patch applied and NATIVE is with or without the patch for !guests. PRE: (gdb) disassemble lock_sock_nested Dump of assembler code for function lock_sock_nested: 0xffffffff817be970 <+0>: push %rbp 0xffffffff817be971 <+1>: mov %rdi,%rbp 0xffffffff817be974 <+4>: push %rbx 0xffffffff817be975 <+5>: lea 0x88(%rbp),%rbx 0xffffffff817be97c <+12>: callq 0xffffffff819f7160 <_cond_resched> 0xffffffff817be981 <+17>: mov %rbx,%rdi 0xffffffff817be984 <+20>: callq 0xffffffff819fbb00 <_raw_spin_lock_bh> 0xffffffff817be989 <+25>: mov 0x8c(%rbp),%eax 0xffffffff817be98f <+31>: test %eax,%eax 0xffffffff817be991 <+33>: jne 0xffffffff817be9ba <lock_sock_nested+74> 0xffffffff817be993 <+35>: movl $0x1,0x8c(%rbp) 0xffffffff817be99d <+45>: mov %rbx,%rdi 0xffffffff817be9a0 <+48>: callq *0xffffffff822299e8 0xffffffff817be9a7 <+55>: pop %rbx 0xffffffff817be9a8 <+56>: pop %rbp 0xffffffff817be9a9 <+57>: mov $0x200,%esi 0xffffffff817be9ae <+62>: mov $0xffffffff817be993,%rdi 0xffffffff817be9b5 <+69>: jmpq 0xffffffff81063ae0 <__local_bh_enable_ip> 0xffffffff817be9ba <+74>: mov %rbp,%rdi 0xffffffff817be9bd <+77>: callq 0xffffffff817be8c0 <__lock_sock> 0xffffffff817be9c2 <+82>: jmp 0xffffffff817be993 <lock_sock_nested+35> End of assembler dump. POST: (gdb) disassemble lock_sock_nested Dump of assembler code for function lock_sock_nested: 0xffffffff817be970 <+0>: push %rbp 0xffffffff817be971 <+1>: mov %rdi,%rbp 0xffffffff817be974 <+4>: push %rbx 0xffffffff817be975 <+5>: lea 0x88(%rbp),%rbx 0xffffffff817be97c <+12>: callq 0xffffffff819f7160 <_cond_resched> 0xffffffff817be981 <+17>: mov %rbx,%rdi 0xffffffff817be984 <+20>: callq 0xffffffff819fbb00 <_raw_spin_lock_bh> 0xffffffff817be989 <+25>: mov 0x8c(%rbp),%eax 0xffffffff817be98f <+31>: test %eax,%eax 0xffffffff817be991 <+33>: jne 0xffffffff817be9ba <lock_sock_nested+74> 0xffffffff817be993 <+35>: movl $0x1,0x8c(%rbp) 0xffffffff817be99d <+45>: mov %rbx,%rdi 0xffffffff817be9a0 <+48>: callq 0xffffffff810a0c20 <__raw_callee_save___pv_queued_spin_unlock> 0xffffffff817be9a5 <+53>: xchg %ax,%ax 0xffffffff817be9a7 <+55>: pop %rbx 0xffffffff817be9a8 <+56>: pop %rbp 0xffffffff817be9a9 <+57>: mov $0x200,%esi 0xffffffff817be9ae <+62>: mov $0xffffffff817be993,%rdi 0xffffffff817be9b5 <+69>: jmpq 0xffffffff81063aa0 <__local_bh_enable_ip> 0xffffffff817be9ba <+74>: mov %rbp,%rdi 0xffffffff817be9bd <+77>: callq 0xffffffff817be8c0 <__lock_sock> 0xffffffff817be9c2 <+82>: jmp 0xffffffff817be993 <lock_sock_nested+35> End of assembler dump. NATIVE: (gdb) disassemble lock_sock_nested Dump of assembler code for function lock_sock_nested: 0xffffffff817be970 <+0>: push %rbp 0xffffffff817be971 <+1>: mov %rdi,%rbp 0xffffffff817be974 <+4>: push %rbx 0xffffffff817be975 <+5>: lea 0x88(%rbp),%rbx 0xffffffff817be97c <+12>: callq 0xffffffff819f7160 <_cond_resched> 0xffffffff817be981 <+17>: mov %rbx,%rdi 0xffffffff817be984 <+20>: callq 0xffffffff819fbb00 <_raw_spin_lock_bh> 0xffffffff817be989 <+25>: mov 0x8c(%rbp),%eax 0xffffffff817be98f <+31>: test %eax,%eax 0xffffffff817be991 <+33>: jne 0xffffffff817be9ba <lock_sock_nested+74> 0xffffffff817be993 <+35>: movl $0x1,0x8c(%rbp) 0xffffffff817be99d <+45>: mov %rbx,%rdi 0xffffffff817be9a0 <+48>: movb $0x0,(%rdi) 0xffffffff817be9a3 <+51>: nopl 0x0(%rax) 0xffffffff817be9a7 <+55>: pop %rbx 0xffffffff817be9a8 <+56>: pop %rbp 0xffffffff817be9a9 <+57>: mov $0x200,%esi 0xffffffff817be9ae <+62>: mov $0xffffffff817be993,%rdi 0xffffffff817be9b5 <+69>: jmpq 0xffffffff81063ae0 <__local_bh_enable_ip> 0xffffffff817be9ba <+74>: mov %rbp,%rdi 0xffffffff817be9bd <+77>: callq 0xffffffff817be8c0 <__lock_sock> 0xffffffff817be9c2 <+82>: jmp 0xffffffff817be993 <lock_sock_nested+35> End of assembler dump. Fixes: 63f7027 ("[PATCH] i386: PARAVIRT: add common patching machinery") Fixes: 3010a06 ("x86/paravirt, objtool: Annotate indirect calls") Reported-by: Nadav Amit <namit@vmware.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Reviewed-by: Juergen Gross <jgross@suse.com> Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com> Cc: David Woodhouse <dwmw2@infradead.org> Cc: stable@vger.kernel.org
diff --git a/arch/x86/kernel/paravirt.c b/arch/x86/kernel/paravirt.c
@@ -88,10 +88,12 @@ unsigned paravirt_patch_call(void *insnbuf,
 	struct branch *b = insnbuf;
 	unsigned long delta = (unsigned long)target - (addr+5);
 
-	if (tgt_clobbers & ~site_clobbers)
-		return len;	/* target would clobber too much for this site */
-	if (len < 5)
+	if (len < 5) {
+#ifdef CONFIG_RETPOLINE
+		WARN_ONCE("Failing to patch indirect CALL in %ps\n", (void *)addr);
+#endif
 		return len;	/* call too long for patch site */
+	}
 
 	b->opcode = 0xe8; /* call */
 	b->delta = delta;
@@ -106,8 +108,12 @@ unsigned paravirt_patch_jmp(void *insnbuf, const void *target,
 	struct branch *b = insnbuf;
 	unsigned long delta = (unsigned long)target - (addr+5);
 
-	if (len < 5)
+	if (len < 5) {
+#ifdef CONFIG_RETPOLINE
+		WARN_ONCE("Failing to patch indirect JMP in %ps\n", (void *)addr);
+#endif
 		return len;	/* call too long for patch site */
+	}
 
 	b->opcode = 0xe9;	/* jmp */
 	b->delta = delta;
