x86/optprobe: Fix OPTPROBE vs UACCESS

While looking at an objtool UACCESS warning, it suddenly occurred to me
that it is entirely possible to have an OPTPROBE right in the middle of
an UACCESS region.

In this case we must of course clear FLAGS.AC while running the KPROBE.
Luckily the trampoline already saves/restores [ER]FLAGS, so all we need
to do is inject a CLAC. Unfortunately we cannot use ALTERNATIVE() in the
trampoline text, so we have to frob that manually.

Fixes: ca0bbc70f147 ("sched/x86_64: Don't save flags on context switch")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Link: https://lkml.kernel.org/r/20200305092130.GU2596@hirez.programming.kicks-ass.net

diff --git a/arch/x86/include/asm/kprobes.h b/arch/x86/include/asm/kprobes.h
index 95b1f053bd96d0080a9693c7c88da8dcf84e00bf..073eb7ad2f56941eb62d866972c882f6cd2ff752 100644 (file)
--- a/arch/x86/include/asm/kprobes.h
+++ b/arch/x86/include/asm/kprobes.h
@@ -36,6 +36,7 @@ typedef u8 kprobe_opcode_t;
 
 /* optinsn template addresses */
 extern __visible kprobe_opcode_t optprobe_template_entry[];
+extern __visible kprobe_opcode_t optprobe_template_clac[];
 extern __visible kprobe_opcode_t optprobe_template_val[];
 extern __visible kprobe_opcode_t optprobe_template_call[];
 extern __visible kprobe_opcode_t optprobe_template_end[];

diff --git a/arch/x86/kernel/kprobes/opt.c b/arch/x86/kernel/kprobes/opt.c
index 3f45b5c43a71c51042250b7354cbfb962019579b..ea13f688828498da37dbc722b887aa93845a1abe 100644 (file)
--- a/arch/x86/kernel/kprobes/opt.c
+++ b/arch/x86/kernel/kprobes/opt.c
@@ -71,6 +71,21 @@ found:
        return (unsigned long)buf;
 }
 
+static void synthesize_clac(kprobe_opcode_t *addr)
+{
+       /*
+        * Can't be static_cpu_has() due to how objtool treats this feature bit.
+        * This isn't a fast path anyway.
+        */
+       if (!boot_cpu_has(X86_FEATURE_SMAP))
+               return;
+
+       /* Replace the NOP3 with CLAC */
+       addr[0] = 0x0f;
+       addr[1] = 0x01;
+       addr[2] = 0xca;
+}
+
 /* Insert a move instruction which sets a pointer to eax/rdi (1st arg). */
 static void synthesize_set_arg1(kprobe_opcode_t *addr, unsigned long val)
 {
@@ -92,6 +107,9 @@ asm (
                        /* We don't bother saving the ss register */
                        "       pushq %rsp\n"
                        "       pushfq\n"
+                       ".global optprobe_template_clac\n"
+                       "optprobe_template_clac:\n"
+                       ASM_NOP3
                        SAVE_REGS_STRING
                        "       movq %rsp, %rsi\n"
                        ".global optprobe_template_val\n"
@@ -111,6 +129,9 @@ asm (
 #else /* CONFIG_X86_32 */
                        "       pushl %esp\n"
                        "       pushfl\n"
+                       ".global optprobe_template_clac\n"
+                       "optprobe_template_clac:\n"
+                       ASM_NOP3
                        SAVE_REGS_STRING
                        "       movl %esp, %edx\n"
                        ".global optprobe_template_val\n"
@@ -134,6 +155,8 @@ asm (
 void optprobe_template_func(void);
 STACK_FRAME_NON_STANDARD(optprobe_template_func);
 
+#define TMPL_CLAC_IDX \
+       ((long)optprobe_template_clac - (long)optprobe_template_entry)
 #define TMPL_MOVE_IDX \
        ((long)optprobe_template_val - (long)optprobe_template_entry)
 #define TMPL_CALL_IDX \
@@ -389,6 +412,8 @@ int arch_prepare_optimized_kprobe(struct optimized_kprobe *op,
        op->optinsn.size = ret;
        len = TMPL_END_IDX + op->optinsn.size;
 
+       synthesize_clac(buf + TMPL_CLAC_IDX);
+
        /* Set probe information */
        synthesize_set_arg1(buf + TMPL_MOVE_IDX, (unsigned long)op);