[RFC PATCH v2] fs/xattr: add *at family syscalls

[Amir Goldstein]

On Mon, May 15, 2023 at 1:33 PM Christian Brauner <brauner at kernel.org> wrote:
>
> On Thu, May 11, 2023 at 05:08:02PM +0200, Christian Göttsche wrote:
> > Add the four syscalls setxattrat(), getxattrat(), listxattrat() and
> > removexattrat().  Those can be used to operate on extended attributes,
> > especially security related ones, either relative to a pinned directory
> > or on a file descriptor without read access, avoiding a
> > /proc/<pid>/fd/<fd> detour, requiring a mounted procfs.
> >
> > One use case will be setfiles(8) setting SELinux file contexts
> > ("security.selinux") without race conditions.
> >
> > Add XATTR flags to the private namespace of AT_* flags.
> >
> > Use the do_{name}at() pattern from fs/open.c.
> >
> > Use a single flag parameter for extended attribute flags (currently
> > XATTR_CREATE and XATTR_REPLACE) and *at() flags to not exceed six
> > syscall arguments in setxattrat().
> >
> > Previous approach ("f*xattr: allow O_PATH descriptors"): https://lore.kernel.org/all/20220607153139.35588-1-cgzones@googlemail.com/
> > v1 discussion: https://lore.kernel.org/all/20220830152858.14866-2-cgzones@googlemail.com/
> >
> > Signed-off-by: Christian Göttsche <cgzones at googlemail.com>
> > CC: x86 at kernel.org
> > CC: linux-alpha at vger.kernel.org
> > CC: linux-kernel at vger.kernel.org
> > CC: linux-arm-kernel at lists.infradead.org
> > CC: linux-ia64 at vger.kernel.org
> > CC: linux-m68k at lists.linux-m68k.org
> > CC: linux-mips at vger.kernel.org
> > CC: linux-parisc at vger.kernel.org
> > CC: linuxppc-dev at lists.ozlabs.org
> > CC: linux-s390 at vger.kernel.org
> > CC: linux-sh at vger.kernel.org
> > CC: sparclinux at vger.kernel.org
> > CC: linux-fsdevel at vger.kernel.org
> > CC: audit at vger.kernel.org
> > CC: linux-arch at vger.kernel.org
> > CC: linux-api at vger.kernel.org
> > CC: linux-security-module at vger.kernel.org
> > CC: selinux at vger.kernel.org
> > ---
>
> Fwiw, your header doesn't let me see who the mail was directly sent to
> so I'm only able to reply to lists which is a bit pointless...
>
> > v2:
> >   - squash syscall introduction and wire up commits
> >   - add AT_XATTR_CREATE and AT_XATTR_REPLACE constants
>
> > +#define AT_XATTR_CREATE              0x1     /* setxattrat(2): set value, fail if attr already exists */
> > +#define AT_XATTR_REPLACE     0x2     /* setxattrat(2): set value, fail if attr does not exist */
>
> We really shouldn't waste any AT_* flags for this. Otherwise we'll run
> out of them rather quickly. Two weeks ago we added another AT_* flag
> which is up for merging for v6.5 iirc and I've glimpsed another AT_*
> flag proposal in one of the talks at last weeks Vancouver conference
> extravaganza.
>
> Even if we reuse 0x200 for AT_XATTR_CREATE (like we did for AT_EACCESS
> and AT_REMOVEDIR) we still need another bit for AT_XATTR_REPLACE.
>
> Plus, this is really ugly since AT_XATTR_{CREATE,REPLACE} really isn't
> in any way related to lookup and we're mixing it in with lookup
> modifying flags.
>
> So my proposal for {g,s}etxattrat() would be:
>
> struct xattr_args {
>         __aligned_u64 value;
>         __u32 size;
>         __u32 cmd;
> };
>
> So everything's nicely 64bit aligned in the struct. Use the @cmd member
> to set either XATTR_REPLACE or XATTR_CREATE and treat it as a proper
> enum and not as a flag argument like the old calls did.
>
> So then we'd have:
>
> setxattrat(int dfd, const char *path, const char __user *name,
>            struct xattr_args __user *args, size_t size, unsigned int flags)
> getxattrat(int dfd, const char *path, const char __user *name,
>            struct xattr_args __user *args, size_t size, unsigned int flags)
>
> The current in-kernel struct xattr_ctx would be renamed to struct
> kernel_xattr_args and then we do the usual copy_struct_from_user()
> dance:
>
> struct xattr_args args;
> err = copy_struct_from_user(&args, sizeof(args), uargs, usize);
>
> and then go on to handle value/size for setxattrat()/getxattrat()
> accordingly.
>
> getxattr()/setxattr() aren't meaningfully filterable by seccomp already
> so there's not point in not using a struct.
>
> If that isn't very appealing then another option is to add a new flag
> namespace just for setxattrat() similar to fspick() and move_mount()
> duplicating the needed lookup modifying flags.
> Thoughts?

Here is a thought: I am not sure if I am sorry we did not discuss this API
issue in LSFMM or happy that we did not waste our time on this... :-/

I must say that I dislike redefined flag namespace like FSPICK_*
just as much as I dislike overloading the AT_* namespace and TBH,
I am not crazy about avoiding this problem with xattr_args either.

A more sane solution IMO could have been:
- Use lower word of flags for generic AT_ flags
- Use the upper word of flags for syscall specific flags

So if it were up to me, I would vote starting this practice:

+ /* Start of syscall specific range */
+ #define AT_XATTR_CREATE       0x10000     /* setxattrat(2): set
value, fail if attr already exists */
+ #define AT_XATTR_REPLACE     0x20000     /* setxattrat(2): set
value, fail if attr does not exist */

Which coincidentally happens to be inline with my AT_HANDLE_FID patch...

Sure, we will have some special cases like MOVE_MOUNT_* and
legacy pollution to the lower AT_ flags word, but as a generic solution
for syscalls that need the common AT_ lookup flags and just a few
private flags, that seems like the lesser evil to me.

Thanks,
Amir.