[PATCH] coredump: Do not lock during 'comm' reporting

[Kees Cook]

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 <vegard.nossum@xxxxxxxxxx>
Closes: https://lore.kernel.org/all/d122ece6-3606-49de-ae4d-8da88846bef2@xxxxxxxxxx
Fixes: c114e9948c2b ("coredump: Standartize and fix logging")
Signed-off-by: Kees Cook <kees@xxxxxxxxxx>
---
Cc: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
Cc: Allen Pais <apais@xxxxxxxxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Roman Kisel <romank@xxxxxxxxxxxxxxxxxxx>
Cc: Xiaoming Ni <nixiaoming@xxxxxxxxxx>

Vegard, can you validate that this fixes the problem for you? I have
been wrecked by covid, so very slow to respond here. There's a related
thread about this locked, but we can just totally bypass it here.

---
 include/linux/coredump.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/coredump.h b/include/linux/coredump.h
index edeb8532ce0f..a99079115a38 100644
--- a/include/linux/coredump.h
+++ b/include/linux/coredump.h
@@ -52,8 +52,8 @@ extern int 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)	\
-- 
2.34.1