coredump: Do not lock during 'comm' reporting

The 'comm' member will always be NUL terminated, and this is not
fast-path, so we can just perform a direct memcpy during a coredump
instead of potentially deadlocking while holding the task struct lock.

Reported-by: Vegard Nossum <[email protected]>
Closes: https://lore.kernel.org/all/[email protected]
Fixes: c114e99 ("coredump: Standartize and fix logging")
Tested-by: Vegard Nossum <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Kees Cook <[email protected]>

Author: kees

include/linux/coredump.h

@@ -52,8 +52,8 @@ extern void do_coredump(const kernel_siginfo_t *siginfo);
 #define __COREDUMP_PRINTK(Level, Format, ...) \
 	do {	\
 		char comm[TASK_COMM_LEN];	\
-	\
-		get_task_comm(comm, current);	\
+		/* This will always be NUL terminated. */ \
+		memcpy(comm, current->comm, sizeof(comm)); \
 		printk_ratelimited(Level "coredump: %d(%*pE): " Format "\n",	\
 			task_tgid_vnr(current), (int)strlen(comm), comm, ##__VA_ARGS__);	\
 	} while (0)	\