[PATCH 07/18] fs_context: fix double free of legacy_fs_context data

[Eric Biggers]

From: Eric Biggers <ebiggers@xxxxxxxxxx>

sys_fsmount() calls fc->ops->free() to free the data, zeroes
->fs_private, then proceeds to reuse the context.  But legacy_fs_context
doesn't use ->fs_private, so we need to handle zeroing it too; otherwise
there's a double free of legacy_fs_context::{legacy_data,secdata}.

Fixes: 8a2e54b8af88 ("vfs: Implement a filesystem superblock creation/configuration context")
Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>
---
 fs/fs_context.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/fs_context.c b/fs/fs_context.c
index 7a8d1ed34ae71..206256be9777e 100644
--- a/fs/fs_context.c
+++ b/fs/fs_context.c
@@ -504,6 +504,9 @@ static void legacy_fs_context_free(struct fs_context *fc)
 		kfree(ctx->legacy_data);
 		break;
 	}
+	/* This doesn't use fs_private, so need to manually zero for fsmount */
+	BUILD_BUG_ON(offsetof(struct legacy_fs_context, fc) != 0);
+	memset(fc + 1, 0, sizeof(*ctx) - sizeof(*fc));
 }
 
 /*
-- 
2.18.0