[PATCH v2] tty: vt: fix memory leak in vc_allocate()

[Mingyu Wang]

If the screen buffer allocation fails in vc_allocate(), the error handling
path jumps to `err_free`. However, this path fails to release the unicode
screen map attached to `vc->uni_pagedict_loc`.

During the early stages of vc_allocate(), the unicode screen map is either
newly allocated via con_set_default_unimap() or shares the default unicode
map from a previously initialized console (which increments its refcount).
If the subsequent kzalloc() for the screen buffer fails, the err_free path
frees the vc structure but leaves the attached uni_pagedict with an
elevated refcount. This results in an unreferenced object memory leak, as
the reference to the dictionary is lost and its refcount can never reach
zero.

This issue was discovered by DevGen (an automated virtual device modeling
fuzzer based on Syzkaller). During fuzzing with kernel fault injection
(failslab) enabled, the fuzzer forcefully failed the kzalloc() for the
screen buffer, exposing this error-handling path leak.

Fix this by calling con_free_unimap(vc) in the err_free path before
kfree(vc). This safely decrements the refcount and releases the dictionary
memory if this was the last reference.

Fixes: 34902b7f2754 ("tty: vt, get rid of weird source code flow")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Mingyu Wang <25181214217@xxxxxxxxxxxxxxxxx>
---
Changes in v2:
- Added documentation in the commit message detailing the fuzzing
  environment (DevGen/Syzkaller) and the fault injection (failslab)
  mechanism used to discover the bug, as requested by Greg KH.

 drivers/tty/vt/vt.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
index 8f467b22b799..4bac89ea968a 100644
--- a/drivers/tty/vt/vt.c
+++ b/drivers/tty/vt/vt.c
@@ -1134,6 +1134,7 @@ int vc_allocate(unsigned int currcons)	/* return 0 on success */
 	return 0;
 err_free:
 	visual_deinit(vc);
+	con_free_unimap(vc);
 	kfree(vc);
 	vc_cons[currcons].d = NULL;
 	return err;
-- 
2.34.1