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

The local_readlink() callback is vulnerable to symlink attacks because it
calls:

(1) open(O_NOFOLLOW) which follows symbolic links for all path elements but
    the rightmost one
(2) readlink() which follows symbolic links for all path elements but the
    rightmost one

This patch converts local_readlink() to rely on open_nofollow() to fix (1)
and opendir_nofollow(), readlinkat() to fix (2).

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 23:43:40 +01:00
parent ac125d993b
commit bec1e9546e
1 changed files with 17 additions and 9 deletions
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26
hw/9pfs/9p-local.c
View File

@ -340,27 +340,35 @@ static ssize_t local_readlink(FsContext *fs_ctx, V9fsPath *fs_path,
char *buf, size_t bufsz)
{
ssize_t tsize = -1;
char *buffer;
char *path = fs_path->data;
if ((fs_ctx->export_flags & V9FS_SM_MAPPED) ||
(fs_ctx->export_flags & V9FS_SM_MAPPED_FILE)) {
int fd;
buffer = rpath(fs_ctx, path);
fd = open(buffer, O_RDONLY | O_NOFOLLOW);
g_free(buffer);
fd = local_open_nofollow(fs_ctx, fs_path->data, O_RDONLY, 0);
if (fd == -1) {
return -1;
}
do {
tsize = read(fd, (void *)buf, bufsz);
} while (tsize == -1 && errno == EINTR);
close(fd);
close_preserve_errno(fd);
} else if ((fs_ctx->export_flags & V9FS_SM_PASSTHROUGH) ||
(fs_ctx->export_flags & V9FS_SM_NONE)) {
buffer = rpath(fs_ctx, path);
tsize = readlink(buffer, buf, bufsz);
g_free(buffer);
char *dirpath = g_path_get_dirname(fs_path->data);
char *name = g_path_get_basename(fs_path->data);
int dirfd;
dirfd = local_opendir_nofollow(fs_ctx, dirpath);
if (dirfd == -1) {
goto out;
}
tsize = readlinkat(dirfd, name, buf, bufsz);
close_preserve_errno(dirfd);
out:
g_free(name);
g_free(dirpath);
}
return tsize;
}