x86/dumpstack: Improve opcodes dumping in the code section

The code used to iterate byte-by-byte over the bytes around RIP and that
is expensive: disabling pagefaults around it, copy_from_user, etc...

Make it read the whole buffer of OPCODE_BUFSIZE size in one go. Use a
statically allocated 64 bytes buffer so that concurrent show_opcodes()
do not interleave in the output even though in the majority of the cases
it's serialized via die_lock. Except the #PF path which doesn't...

Also, do the PAGE_OFFSET check outside of the function because latter
will be reused in other context.

Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Link: https://lkml.kernel.org/r/20180417161124.5294-5-bp@alien8.de

diff --git a/arch/x86/kernel/dumpstack.c b/arch/x86/kernel/dumpstack.c
index eb9d6c00a52f1597223fd70a50ff6be75bdb2515..1d6698b545279e5232fbb2742bf725dbca723724 100644 (file)
--- a/arch/x86/kernel/dumpstack.c
+++ b/arch/x86/kernel/dumpstack.c
@@ -72,29 +72,24 @@ static void printk_stack_address(unsigned long address, int reliable,
 
 static void show_opcodes(u8 *rip)
 {
-       unsigned int code_prologue = OPCODE_BUFSIZE * 43 / 64;
-       unsigned int code_len = OPCODE_BUFSIZE;
-       unsigned char c;
+       unsigned int code_prologue = OPCODE_BUFSIZE * 2 / 3;
+       u8 opcodes[OPCODE_BUFSIZE];
        u8 *ip;
        int i;
 
        printk(KERN_DEFAULT "Code: ");
 
        ip = (u8 *)rip - code_prologue;
-       if (ip < (u8 *)PAGE_OFFSET || probe_kernel_address(ip, c)) {
-               /* try starting at IP */
-               ip = (u8 *)rip;
-               code_len = code_len - code_prologue + 1;
+       if (probe_kernel_read(opcodes, ip, OPCODE_BUFSIZE)) {
+               pr_cont("Bad RIP value.\n");
+               return;
        }
-       for (i = 0; i < code_len; i++, ip++) {
-               if (ip < (u8 *)PAGE_OFFSET || probe_kernel_address(ip, c)) {
-                       pr_cont(" Bad RIP value.");
-                       break;
-               }
-               if (ip == (u8 *)rip)
-                       pr_cont("<%02x> ", c);
+
+       for (i = 0; i < OPCODE_BUFSIZE; i++, ip++) {
+               if (ip == rip)
+                       pr_cont("<%02x> ", opcodes[i]);
                else
-                       pr_cont("%02x ", c);
+                       pr_cont("%02x ", opcodes[i]);
        }
        pr_cont("\n");
 }
@@ -402,6 +397,10 @@ void show_regs(struct pt_regs *regs)
         */
        if (!user_mode(regs)) {
                show_trace_log_lvl(current, regs, NULL, KERN_DEFAULT);
-               show_opcodes((u8 *)regs->ip);
+
+               if (regs->ip < PAGE_OFFSET)
+                       printk(KERN_DEFAULT "Code: Bad RIP value.\n");
+               else
+                       show_opcodes((u8 *)regs->ip);
        }
 }