[PATCH] ring buffer: propagate __rb_map_vma return value to caller

[Ankit Khushwaha <ankitkhushwaha.linux@gmail.com>]

The return value from `__rb_map_vma()`, which rejects writable or
executable mappings (VM_WRITE, VM_EXEC, or !VM_MAYSHARE), was being
ignored. As a result the caller of `__rb_map_vma` always returned 0 
even when the mapping had actually failed, allowing it to proceed
with an invalid VMA.

Reported-by: syzbot+ddc001b92c083dbf2b97@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?id=194151be8eaebd826005329b2e123aecae714bdb
Signed-off-by: Ankit Khushwaha <ankitkhushwaha.linux@gmail.com>
---

#syz test: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git 4ff71af020ae

---
 kernel/trace/ring_buffer.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 43460949ad3f..4efb90364f48 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -7271,6 +7271,8 @@ int ring_buffer_map(struct trace_buffer *buffer, int cpu,
 		cpu_buffer->subbuf_ids = NULL;
 		rb_free_meta_page(cpu_buffer);
 		atomic_dec(&cpu_buffer->resize_disabled);
+		/* VM failed to be mapped */
+		return err;
 	}
 
 	return 0;
-- 
2.51.0