Re: [PATCH] Smack: Provide read control for io_uring_cmd

[Joel Granados <j.granados@samsung.com>]

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Hey Casey

I have tested this patch and I see the smack_uring_cmd prevents access
to file when smack labels are different. They way I got there was a bit
convoluted and I'll describe it here so ppl can judge how valid the test
is.

Tested-by: Joel Granados <j.granados@samsung.com>

I started by reproducing what Kanchan had done by changing the smack
label from "_" to "Snap". Then I ran the io_uring passthrough test
included in liburing with an unprivileged user and saw that the
smack_uring_cmd function was NOT executed because smack prevented an open on
the device file ("/dev/ng0n1" on my box).

So here I got a bit sneaky and changed the label after the open to the
device was done. This is how I did it:
1. In the io_uring_passthrough.c tests I stopped execution after the
   device was open and before the actual IO.
2. While on hold I changed the label of the device from "_" to "Snap".
   At this point the device had a "Snap" label and the executable had a
   "_" label.
3. Previous to execution I had placed a printk inside the
   smack_uring_cmd function. And I also made sure to printk whenever
   security_uring_command returned because of a security violation.
4. I continued execution and saw that both smack_uiring_cmd and
   io_uring_cmd returned error. Which is what I expected.

I'm still a bit unsure of my setup so if anyone has comments or a way to
make it better, I would really appreciate feedback.

Best

Joel

On Mon, Aug 29, 2022 at 09:20:09AM -0700, Casey Schaufler wrote:
> On 8/27/2022 8:59 AM, Kanchan Joshi wrote:
> > On Tue, Aug 23, 2022 at 04:46:18PM -0700, Casey Schaufler wrote:
> >> Limit io_uring "cmd" options to files for which the caller has
> >> Smack read access. There may be cases where the cmd option may
> >> be closer to a write access than a read, but there is no way
> >> to make that determination.
> >>
> >> Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>
> >> -- 
> >> security/smack/smack_lsm.c | 32 ++++++++++++++++++++++++++++++++
> >> 1 file changed, 32 insertions(+)
> >>
> >> diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
> >> index 001831458fa2..bffccdc494cb 100644
> >> --- a/security/smack/smack_lsm.c
> >> +++ b/security/smack/smack_lsm.c
> >> @@ -42,6 +42,7 @@
> >> #include <linux/fs_context.h>
> >> #include <linux/fs_parser.h>
> >> #include <linux/watch_queue.h>
> >> +#include <linux/io_uring.h>
> >> #include "smack.h"
> >>
> >> #define TRANS_TRUE    "TRUE"
> >> @@ -4732,6 +4733,36 @@ static int smack_uring_sqpoll(void)
> >>     return -EPERM;
> >> }
> >>
> >> +/**
> >> + * smack_uring_cmd - check on file operations for io_uring
> >> + * @ioucmd: the command in question
> >> + *
> >> + * Make a best guess about whether a io_uring "command" should
> >> + * be allowed. Use the same logic used for determining if the
> >> + * file could be opened for read in the absence of better criteria.
> >> + */
> >> +static int smack_uring_cmd(struct io_uring_cmd *ioucmd)
> >> +{
> >> +    struct file *file = ioucmd->file;
> >> +    struct smk_audit_info ad;
> >> +    struct task_smack *tsp;
> >> +    struct inode *inode;
> >> +    int rc;
> >> +
> >> +    if (!file)
> >> +        return -EINVAL;
> >> +
> >> +    tsp = smack_cred(file->f_cred);
> >> +    inode = file_inode(file);
> >> +
> >> +    smk_ad_init(&ad, __func__, LSM_AUDIT_DATA_PATH);
> >> +    smk_ad_setfield_u_fs_path(&ad, file->f_path);
> >> +    rc = smk_tskacc(tsp, smk_of_inode(inode), MAY_READ, &ad);
> >> +    rc = smk_bu_credfile(file->f_cred, file, MAY_READ, rc);
> >> +
> >> +    return rc;
> >> +}
> >> +
> >> #endif /* CONFIG_IO_URING */
> >>
> >> struct lsm_blob_sizes smack_blob_sizes __lsm_ro_after_init = {
> >> @@ -4889,6 +4920,7 @@ static struct security_hook_list smack_hooks[]
> >> __lsm_ro_after_init = {
> >> #ifdef CONFIG_IO_URING
> >>     LSM_HOOK_INIT(uring_override_creds, smack_uring_override_creds),
> >>     LSM_HOOK_INIT(uring_sqpoll, smack_uring_sqpoll),
> >> +    LSM_HOOK_INIT(uring_cmd, smack_uring_cmd),
> >> #endif
> >
> > Tried this on nvme device (/dev/ng0n1).
> > Took a while to come out of noob setup-related issues but I see that
> > smack is listed (in /sys/kernel/security/lsm), smackfs is present, and
> > the hook (smack_uring_cmd) gets triggered fine on doing I/O on
> > /dev/ng0n1.
> >
> > I/O goes fine, which seems aligned with the label on /dev/ng0n1 (which
> > is set to floor).
> >
> > $ chsmack -L /dev/ng0n1
> > /dev/ng0n1 access="_"
> 
> Setting the Smack on the object that the cmd operates on to
> something other than "_" would be the correct test. If that
> is /dev/ng0n1 you could use
> 
> 	# chsmack -a Snap /dev/ng0n1
> 
> The unprivileged user won't be able to read /dev/ng0n1 so you
> won't get as far as testing the cmd interface. I don't know
> io_uring and nvme well enough to know what other objects may
> be involved. Noob here, too.
> 
> >
> > I ran fio (/usr/bin/fio), which also has the same label.
> > Hope you expect the same outcome.
> >
> > Do you run something else to see that things are fine e.g. for
> > /dev/null, which also has the same label "_".
> > If yes, I can try the same on nvme side.
> >

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