[PATCH 4/9] x86/dumpstack: Improve opcodes dumping in the Code: section

Borislav Petkov
2018-04-17T16:13:43Z

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
we sync on 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>
---
arch/x86/kernel/dumpstack.c | 31 +++++++++++++++----------------
1 file changed, 15 insertions(+), 16 deletions(-)

diff --git a/arch/x86/kernel/dumpstack.c b/arch/x86/kernel/dumpstack.c
index eb9d6c00a52f..1d6698b54527 100644
--- 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);
}
}
--
2.13.0