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9pfs: local: llistxattr: don't follow symlinks 

The local_llistxattr() callback is vulnerable to symlink attacks because
it calls llistxattr() which follows symbolic links in all path elements but
the rightmost one.

This patch introduces a helper to emulate the non-existing flistxattrat()
function: it is implemented with /proc/self/fd which provides a trusted
path that can be safely passed to llistxattr().

local_llistxattr() is converted to use this helper and opendir_nofollow().

This partly fixes CVE-2016-9602.

Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
This commit is contained in:
Greg Kurz 2017-02-26 23:42:34 +01:00
parent 56ad3e54da
commit 5507904e36
1 changed files with 29 additions and 6 deletions
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35
hw/9pfs/9p-xattr.c
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@ -60,6 +60,16 @@ ssize_t pt_listxattr(FsContext *ctx, const char *path,
return name_size;
}
static ssize_t flistxattrat_nofollow(int dirfd, const char *filename,
char *list, size_t size)
{
char *proc_path = g_strdup_printf("/proc/self/fd/%d/%s", dirfd, filename);
int ret;
ret = llistxattr(proc_path, list, size);
g_free(proc_path);
return ret;
}
/*
* Get the list and pass to each layer to find out whether
@ -69,24 +79,37 @@ ssize_t v9fs_list_xattr(FsContext *ctx, const char *path,
void *value, size_t vsize)
{
ssize_t size = 0;
char *buffer;
void *ovalue = value;
XattrOperations *xops;
char *orig_value, *orig_value_start;
ssize_t xattr_len, parsed_len = 0, attr_len;
char *dirpath, *name;
int dirfd;
/* Get the actual len */
buffer = rpath(ctx, path);
xattr_len = llistxattr(buffer, value, 0);
dirpath = g_path_get_dirname(path);
dirfd = local_opendir_nofollow(ctx, dirpath);
g_free(dirpath);
if (dirfd == -1) {
return -1;
}
name = g_path_get_basename(path);
xattr_len = flistxattrat_nofollow(dirfd, name, value, 0);
if (xattr_len <= 0) {
g_free(buffer);
g_free(name);
close_preserve_errno(dirfd);
return xattr_len;
}
/* Now fetch the xattr and find the actual size */
orig_value = g_malloc(xattr_len);
xattr_len = llistxattr(buffer, orig_value, xattr_len);
g_free(buffer);
xattr_len = flistxattrat_nofollow(dirfd, name, orig_value, xattr_len);
g_free(name);
close_preserve_errno(dirfd);
if (xattr_len < 0) {
return -1;
}
/* store the orig pointer */
orig_value_start = orig_value;