Re: [PATCH v2 02/30] fuse: Clear setuid bit even in cache=never path

Miklos Szeredi
2019-05-20T14:44:47Z

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On Mon, May 20, 2019 at 04:41:37PM +0200, Miklos Szeredi wrote:
> On Wed, May 15, 2019 at 03:26:47PM -0400, Vivek Goyal wrote:
> > If fuse daemon is started with cache=never, fuse falls back to direct IO.
> > In that write path we don't call file_remove_privs() and that means setuid
> > bit is not cleared if unpriviliged user writes to a file with setuid bit set.
> >
> > pjdfstest chmod test 12.t tests this and fails.
>
> I think better sulution is to tell the server if the suid bit needs to be
> removed, so it can do so in a race free way.
>
> Here's the kernel patch, and I'll reply with the libfuse patch.

Here are the patches for libfuse and passthrough_ll.

[-- Attachment #2: libfuse-add-fuse_write_kill_priv.patch --]
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---
include/fuse_common.h | 5 ++++-
include/fuse_kernel.h | 2 ++
lib/fuse_lowlevel.c | 12 ++++++++----
3 files changed, 14 insertions(+), 5 deletions(-)

--- a/include/fuse_common.h
+++ b/include/fuse_common.h
@@ -64,8 +64,11 @@ struct fuse_file_info {
May only be set in ->release(). */
unsigned int flock_release : 1;

+ /* Kill suid and sgid bits on write */
+ unsigned int write_kill_priv : 1;
+
/** Padding. Do not use*/
- unsigned int padding : 27;
+ unsigned int padding : 26;

/** File handle. May be filled in by filesystem in open().
Available in all other file operations */
--- a/include/fuse_kernel.h
+++ b/include/fuse_kernel.h
@@ -304,9 +304,11 @@ struct fuse_file_lock {
*
* FUSE_WRITE_CACHE: delayed write from page cache, file handle is guessed
* FUSE_WRITE_LOCKOWNER: lock_owner field is valid
+ * FUSE_WRITE_KILL_PRIV: kill suid and sgid bits
*/
#define FUSE_WRITE_CACHE (1 << 0)
#define FUSE_WRITE_LOCKOWNER (1 << 1)
+#define FUSE_WRITE_KILL_PRIV (1 << 2)

/**
* Read flags
--- a/lib/fuse_lowlevel.c
+++ b/lib/fuse_lowlevel.c
@@ -1315,12 +1315,14 @@ static void do_write(fuse_req_t req, fus

memset(&fi, 0, sizeof(fi));
fi.fh = arg->fh;
- fi.writepage = (arg->write_flags & 1) != 0;
+ fi.writepage = (arg->write_flags & FUSE_WRITE_CACHE) != 0;
+ fi.write_kill_priv = (arg->write_flags & FUSE_WRITE_KILL_PRIV) != 0;

if (req->se->conn.proto_minor < 9) {
param = ((char *) arg) + FUSE_COMPAT_WRITE_IN_SIZE;
} else {
- fi.lock_owner = arg->lock_owner;
+ if (arg->write_flags & FUSE_WRITE_LOCKOWNER)
+ fi.lock_owner = arg->lock_owner;
fi.flags = arg->flags;
param = PARAM(arg);
}
@@ -1345,7 +1347,8 @@ static void do_write_buf(fuse_req_t req,

memset(&fi, 0, sizeof(fi));
fi.fh = arg->fh;
- fi.writepage = arg->write_flags & 1;
+ fi.writepage = (arg->write_flags & FUSE_WRITE_CACHE) != 0;
+ fi.write_kill_priv = (arg->write_flags & FUSE_WRITE_KILL_PRIV) != 0;

if (se->conn.proto_minor < 9) {
bufv.buf[0].mem = ((char *) arg) + FUSE_COMPAT_WRITE_IN_SIZE;
@@ -1353,7 +1356,8 @@ static void do_write_buf(fuse_req_t req,
FUSE_COMPAT_WRITE_IN_SIZE;
assert(!(bufv.buf[0].flags & FUSE_BUF_IS_FD));
} else {
- fi.lock_owner = arg->lock_owner;
+ if (arg->write_flags & FUSE_WRITE_LOCKOWNER)
+ fi.lock_owner = arg->lock_owner;
fi.flags = arg->flags;
if (!(bufv.buf[0].flags & FUSE_BUF_IS_FD))
bufv.buf[0].mem = PARAM(arg);

[-- Attachment #3: passthrough_ll-kill-suid.patch --]
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---
example/passthrough_ll.c | 29 ++++++++++++++++++++++++++++-
1 file changed, 28 insertions(+), 1 deletion(-)

--- a/example/passthrough_ll.c
+++ b/example/passthrough_ll.c
@@ -56,6 +56,7 @@
#include <sys/file.h>
#include <sys/xattr.h>
#include <sys/syscall.h>
+#include <sys/capability.h>

/* We are re-using pointers to our `struct lo_inode` and `struct
lo_dirp` elements as inodes. This means that we must be able to
@@ -965,6 +966,11 @@ static void lo_write_buf(fuse_req_t req,
(void) ino;
ssize_t res;
struct fuse_bufvec out_buf = FUSE_BUFVEC_INIT(fuse_buf_size(in_buf));
+ struct __user_cap_header_struct cap_hdr = {
+ .version = _LINUX_CAPABILITY_VERSION_1,
+ };
+ struct __user_cap_data_struct cap_orig;
+ struct __user_cap_data_struct cap_new;

out_buf.buf[0].flags = FUSE_BUF_IS_FD | FUSE_BUF_FD_SEEK;
out_buf.buf[0].fd = fi->fh;
@@ -974,7 +980,28 @@ static void lo_write_buf(fuse_req_t req,
fprintf(stderr, "lo_write(ino=%" PRIu64 ", size=%zd, off=%lu)\n",
ino, out_buf.buf[0].size, (unsigned long) off);

+ if (fi->write_kill_priv) {
+ res = capget(&cap_hdr, &cap_orig);
+ if (res == -1) {
+ fuse_reply_err(req, errno);
+ return;
+ }
+ cap_new = cap_orig;
+ cap_new.effective &= ~(1 << CAP_FSETID);
+ res = capset(&cap_hdr, &cap_new);
+ if (res == -1) {
+ fuse_reply_err(req, errno);
+ return;
+ }
+ }
+
res = fuse_buf_copy(&out_buf, in_buf, 0);
+
+ if (fi->write_kill_priv) {
+ if (capset(&cap_hdr, &cap_orig) != 0)
+ abort();
+ }
+
if(res < 0)
fuse_reply_err(req, -res);
else
@@ -1215,7 +1242,7 @@ static void lo_copy_file_range(fuse_req_
res = copy_file_range(fi_in->fh, &off_in, fi_out->fh, &off_out, len,
flags);
if (res < 0)
- fuse_reply_err(req, -errno);
+ fuse_reply_err(req, errno);
else
fuse_reply_write(req, res);
}