Re: [PATCH] commoncap: check return value to avoid null pointer dereference
From: Al Viro <viro@zeniv.linux.org.uk>
Date: 2022-05-17 21:44:10
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On Mon, May 16, 2022 at 10:40:02AM -0700, Yongzhi Liu wrote:
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The pointer inode is dereferenced before a null pointer check on inode, hence if inode is actually null we will get a null pointer dereference. Fix this by only dereferencing inode after the null pointer check on inode. Fixes: c6f493d631c ("VFS: security/: d_backing_inode() annotations") Fixes: 8db6c34 ("Introduce v3 namespaced file capabilities") Signed-off-by: Yongzhi Liu <redacted> --- security/commoncap.c | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-)diff --git a/security/commoncap.c b/security/commoncap.c index 5fc8986..978f011 100644 --- a/security/commoncap.c +++ b/security/commoncap.c@@ -298,6 +298,8 @@ int cap_inode_need_killpriv(struct dentry *dentry) struct inode *inode = d_backing_inode(dentry); int error; + if (!inode) + return 0; error = __vfs_getxattr(dentry, inode, XATTR_NAME_CAPS, NULL, 0); return error > 0; }
Huh? AFAICS, that has all of two callers - notify_change() and dentry_needs_remove_privs(). Both of them should never be called on negative dentries and both would have blown up well before the call of security_inode_need_killpriv().
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@@ -545,11 +547,13 @@ int cap_convert_nscap(struct user_namespace *mnt_userns, struct dentry *dentry, const struct vfs_cap_data *cap = *ivalue; __u32 magic, nsmagic; struct inode *inode = d_backing_inode(dentry); - struct user_namespace *task_ns = current_user_ns(), - *fs_ns = inode->i_sb->s_user_ns; + struct user_namespace *task_ns = current_user_ns(), *fs_ns; kuid_t rootid; size_t newsize; + if (!inode) + return -EINVAL; + fs_ns = inode->i_sb->s_user_ns; if (!*ivalue) return -EINVAL; if (!validheader(size, cap))
... and that one come from vfs_setxattr(). Again, calling that on a negative dentry is an oopsable bug. If you have any reproducers, please post the stack traces so it would be possible to deal with the underlying problem.