[PATCH 5.10 427/509] tracing: Fix null pointer dereference in tracing_err_log_open()

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From: Mateusz Stachyra <m.stachyra@xxxxxxxxxxx>

commit 02b0095e2fbbc060560c1065f86a211d91e27b26 upstream.

Fix an issue in function 'tracing_err_log_open'.
The function doesn't call 'seq_open' if the file is opened only with
write permissions, which results in 'file->private_data' being left as null.
If we then use 'lseek' on that opened file, 'seq_lseek' dereferences
'file->private_data' in 'mutex_lock(&m->lock)', resulting in a kernel panic.
Writing to this node requires root privileges, therefore this bug
has very little security impact.

Tracefs node: /sys/kernel/tracing/error_log

Example Kernel panic:

Unable to handle kernel NULL pointer dereference at virtual address 0000000000000038
Call trace:
 mutex_lock+0x30/0x110
 seq_lseek+0x34/0xb8
 __arm64_sys_lseek+0x6c/0xb8
 invoke_syscall+0x58/0x13c
 el0_svc_common+0xc4/0x10c
 do_el0_svc+0x24/0x98
 el0_svc+0x24/0x88
 el0t_64_sync_handler+0x84/0xe4
 el0t_64_sync+0x1b4/0x1b8
Code: d503201f aa0803e0 aa1f03e1 aa0103e9 (c8e97d02)
---[ end trace 561d1b49c12cf8a5 ]---
Kernel panic - not syncing: Oops: Fatal exception

Link: https://lore.kernel.org/linux-trace-kernel/20230703155237eucms1p4dfb6a19caa14c79eb6c823d127b39024@eucms1p4
Link: https://lore.kernel.org/linux-trace-kernel/20230704102706eucms1p30d7ecdcc287f46ad67679fc8491b2e0f@eucms1p3

Cc: stable@xxxxxxxxxxxxxxx
Fixes: 8a062902be725 ("tracing: Add tracing error log")
Signed-off-by: Mateusz Stachyra <m.stachyra@xxxxxxxxxxx>
Suggested-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
Acked-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
Signed-off-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 kernel/trace/trace.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -7529,7 +7529,7 @@ static const struct file_operations trac
 	.open           = tracing_err_log_open,
 	.write		= tracing_err_log_write,
 	.read           = seq_read,
-	.llseek         = seq_lseek,
+	.llseek         = tracing_lseek,
 	.release        = tracing_err_log_release,
 };
 





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