The existing code uses min_t(ssize_t, outarg.size, XATTR_LIST_MAX) when parsing the FUSE daemon's response to a zero-length getxattr/listxattr request. On 32-bit kernels, where ssize_t and outarg.size are the same size, this is wrong: The min_t() will pass through any size values that are negative when interpreted as signed. fuse_listxattr() will then return this userspace-supplied negative value, which callers will treat as an error value.
This kind of bug pattern can lead to fairly bad security bugs because of how error codes are used in the Linux kernel. If a caller were to convert the numeric error into an error pointer, like so:
struct foo *func(...) { int len = fuse_getxattr(..., NULL, 0); if (len < 0) return ERR_PTR(len); ... }
then it would end up returning this userspace-supplied negative value cast to a pointer - but the caller of this function wouldn't recognize it as an error pointer (IS_ERR_VALUE() only detects values in the narrow range in which legitimate errno values are), and so it would just be treated as a kernel pointer.
I think there is at least one theoretical codepath where this could happen, but that path would involve virtio-fs with submounts plus some weird SELinux configuration, so I think it's probably not a concern in practice.
Cc: stable@vger.kernel.org Fixes: 63401ccdb2ca ("fuse: limit xattr returned size") Signed-off-by: Jann Horn jannh@google.com --- fs/fuse/xattr.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/fs/fuse/xattr.c b/fs/fuse/xattr.c index 5b423fdbb13f..9f568d345c51 100644 --- a/fs/fuse/xattr.c +++ b/fs/fuse/xattr.c @@ -81,7 +81,7 @@ ssize_t fuse_getxattr(struct inode *inode, const char *name, void *value, } ret = fuse_simple_request(fm, &args); if (!ret && !size) - ret = min_t(ssize_t, outarg.size, XATTR_SIZE_MAX); + ret = min_t(size_t, outarg.size, XATTR_SIZE_MAX); if (ret == -ENOSYS) { fm->fc->no_getxattr = 1; ret = -EOPNOTSUPP; @@ -143,7 +143,7 @@ ssize_t fuse_listxattr(struct dentry *entry, char *list, size_t size) } ret = fuse_simple_request(fm, &args); if (!ret && !size) - ret = min_t(ssize_t, outarg.size, XATTR_LIST_MAX); + ret = min_t(size_t, outarg.size, XATTR_LIST_MAX); if (ret > 0 && size) ret = fuse_verify_xattr_list(list, ret); if (ret == -ENOSYS) {
--- base-commit: b0da640826ba3b6506b4996a6b23a429235e6923 change-id: 20240819-fuse-oob-error-fix-664d082176d5