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

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

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

local_lgetxattr() 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 23:42:26 +01:00
parent 996a0d76d7
commit 56ad3e54da
6 changed files with 43 additions and 28 deletions
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12
hw/9pfs/9p-util.c
View file

@ -11,6 +11,7 @@
*/
#include "qemu/osdep.h"
#include "qemu/xattr.h"
#include "9p-util.h"
int relative_openat_nofollow(int dirfd, const char *path, int flags,
@ -55,3 +56,14 @@ int relative_openat_nofollow(int dirfd, const char *path, int flags,
return fd;
}
ssize_t fgetxattrat_nofollow(int dirfd, const char *filename, const char *name,
void *value, size_t size)
{
char *proc_path = g_strdup_printf("/proc/self/fd/%d/%s", dirfd, filename);
int ret;
ret = lgetxattr(proc_path, name, value, size);
g_free(proc_path);
return ret;
}