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

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

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

local_lsetxattr() 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 23:42:43 +01:00
parent 5507904e36
commit 3e36aba757
5 changed files with 44 additions and 28 deletions
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18
hw/9pfs/9p-posix-acl.c
View File

@ -50,13 +50,8 @@ static ssize_t mp_pacl_listxattr(FsContext *ctx, const char *path,
static int mp_pacl_setxattr(FsContext *ctx, const char *path, const char *name,
void *value, size_t size, int flags)
{
char *buffer;
int ret;
buffer = rpath(ctx, path);
ret = lsetxattr(buffer, MAP_ACL_ACCESS, value, size, flags);
g_free(buffer);
return ret;
return local_setxattr_nofollow(ctx, path, MAP_ACL_ACCESS, value, size,
flags);
}
static int mp_pacl_removexattr(FsContext *ctx,
@ -108,13 +103,8 @@ static ssize_t mp_dacl_listxattr(FsContext *ctx, const char *path,
static int mp_dacl_setxattr(FsContext *ctx, const char *path, const char *name,
void *value, size_t size, int flags)
{
char *buffer;
int ret;
buffer = rpath(ctx, path);
ret = lsetxattr(buffer, MAP_ACL_DEFAULT, value, size, flags);
g_free(buffer);
return ret;
return local_setxattr_nofollow(ctx, path, MAP_ACL_DEFAULT, value, size,
flags);
}
static int mp_dacl_removexattr(FsContext *ctx,

2
hw/9pfs/9p-util.h
View File

@ -48,5 +48,7 @@ int relative_openat_nofollow(int dirfd, const char *path, int flags,
mode_t mode);
ssize_t fgetxattrat_nofollow(int dirfd, const char *path, const char *name,
void *value, size_t size);
int fsetxattrat_nofollow(int dirfd, const char *path, const char *name,
void *value, size_t size, int flags);
#endif

8
hw/9pfs/9p-xattr-user.c
View File

@ -67,9 +67,6 @@ static ssize_t mp_user_listxattr(FsContext *ctx, const char *path,
static int mp_user_setxattr(FsContext *ctx, const char *path, const char *name,
void *value, size_t size, int flags)
{
char *buffer;
int ret;
if (strncmp(name, "user.virtfs.", 12) == 0) {
/*
* Don't allow fetch of user.virtfs namesapce
@ -78,10 +75,7 @@ static int mp_user_setxattr(FsContext *ctx, const char *path, const char *name,
errno = EACCES;
return -1;
}
buffer = rpath(ctx, path);
ret = lsetxattr(buffer, name, value, size, flags);
g_free(buffer);
return ret;
return local_setxattr_nofollow(ctx, path, name, value, size, flags);
}
static int mp_user_removexattr(FsContext *ctx,

41
hw/9pfs/9p-xattr.c
View File

@ -195,16 +195,43 @@ ssize_t pt_getxattr(FsContext *ctx, const char *path, const char *name,
return local_getxattr_nofollow(ctx, path, name, value, size);
}
int fsetxattrat_nofollow(int dirfd, const char *filename, const char *name,
void *value, size_t size, int flags)
{
char *proc_path = g_strdup_printf("/proc/self/fd/%d/%s", dirfd, filename);
int ret;
ret = lsetxattr(proc_path, name, value, size, flags);
g_free(proc_path);
return ret;
}
ssize_t local_setxattr_nofollow(FsContext *ctx, const char *path,
const char *name, void *value, size_t size,
int flags)
{
char *dirpath = g_path_get_dirname(path);
char *filename = g_path_get_basename(path);
int dirfd;
ssize_t ret = -1;
dirfd = local_opendir_nofollow(ctx, dirpath);
if (dirfd == -1) {
goto out;
}
ret = fsetxattrat_nofollow(dirfd, filename, name, value, size, flags);
close_preserve_errno(dirfd);
out:
g_free(dirpath);
g_free(filename);
return ret;
}
int pt_setxattr(FsContext *ctx, const char *path, const char *name, void *value,
size_t size, int flags)
{
char *buffer;
int ret;
buffer = rpath(ctx, path);
ret = lsetxattr(buffer, name, value, size, flags);
g_free(buffer);
return ret;
return local_setxattr_nofollow(ctx, path, name, value, size, flags);
}
int pt_removexattr(FsContext *ctx, const char *path, const char *name)

3
hw/9pfs/9p-xattr.h
View File

@ -31,6 +31,9 @@ typedef struct xattr_operations
ssize_t local_getxattr_nofollow(FsContext *ctx, const char *path,
const char *name, void *value, size_t size);
ssize_t local_setxattr_nofollow(FsContext *ctx, const char *path,
const char *name, void *value, size_t size,
int flags);
extern XattrOperations mapped_user_xattr;
extern XattrOperations passthrough_user_xattr;