Re: [PATCH v6 5/7] fs,doc: Enable to enforce noexec mounts or file exec through O_MAYEXEC

Mickaël Salaün
2020-07-16T14:39:41Z

On 15/07/2020 22:37, Kees Cook wrote:
> On Tue, Jul 14, 2020 at 08:16:36PM +0200, Mickaël Salaün wrote:
>> @@ -2849,7 +2855,7 @@ static int may_open(const struct path *path, int acc_mode, int flag)
>> case S_IFLNK:
>> return -ELOOP;
>> case S_IFDIR:
>> - if (acc_mode & (MAY_WRITE | MAY_EXEC))
>> + if (acc_mode & (MAY_WRITE | MAY_EXEC | MAY_OPENEXEC))
>> return -EISDIR;
>> break;
>
> (I need to figure out where "open for reading" rejects S_IFDIR, since
> it's clearly not here...)
>
>> case S_IFBLK:
>> @@ -2859,13 +2865,26 @@ static int may_open(const struct path *path, int acc_mode, int flag)
>> fallthrough;
>> case S_IFIFO:
>> case S_IFSOCK:
>> - if (acc_mode & MAY_EXEC)
>> + if (acc_mode & (MAY_EXEC | MAY_OPENEXEC))
>> return -EACCES;
>> flag &= ~O_TRUNC;
>> break;
>
> This will immediately break a system that runs code with MAY_OPENEXEC
> set but reads from a block, char, fifo, or socket, even in the case of
> a sysadmin leaving the "file" sysctl disabled.

As documented, O_MAYEXEC is for regular files. The only legitimate use
case seems to be with pipes, which should probably be allowed when
enforcement is disabled.

>
>> case S_IFREG:
>> - if ((acc_mode & MAY_EXEC) && path_noexec(path))
>> - return -EACCES;
>> + if (path_noexec(path)) {
>> + if (acc_mode & MAY_EXEC)
>> + return -EACCES;
>> + if ((acc_mode & MAY_OPENEXEC) &&
>> + (sysctl_open_mayexec_enforce & OPEN_MAYEXEC_ENFORCE_MOUNT))
>> + return -EACCES;
>> + }
>> + if ((acc_mode & MAY_OPENEXEC) &&
>> + (sysctl_open_mayexec_enforce & OPEN_MAYEXEC_ENFORCE_FILE))
>> + /*
>> + * Because acc_mode may change here, the next and only
>> + * use of acc_mode should then be by the following call
>> + * to inode_permission().
>> + */
>> + acc_mode |= MAY_EXEC;
>> break;
>> }
>
> Likely very minor, but I'd like to avoid the path_noexec() call in the
> fast-path (it dereferences a couple pointers where as doing bit tests on
> acc_mode is fast).
>
> Given that and the above observations, I think that may_open() likely
> needs to start with:
>
> if (acc_mode & MAY_OPENEXEC) {
> /* Reject all file types when mount enforcement set. */
> if ((sysctl_open_mayexec_enforce & OPEN_MAYEXEC_ENFORCE_MOUNT) &&
> path_noexec(path))
> return -EACCES;
> /* Treat the same as MAY_EXEC. */
> if (sysctl_open_mayexec_enforce & OPEN_MAYEXEC_ENFORCE_FILE))
> acc_mode |= MAY_EXEC;
> }

OK

>
> (Though I'm not 100% sure that path_noexec() is safe to be called for
> all file types: i.e. path->mnt and path->-mnt->mnt_sb *always* non-NULL?)

path->mnt should always be non-NULL:
https://lore.kernel.org/lkml/20200317164709.GA23230@ZenIV.linux.org.uk/

>
> This change would also imply that OPEN_MAYEXEC_ENFORCE_FILE *includes*
> OPEN_MAYEXEC_ENFORCE_MOUNT (i.e. the sysctl should not be a bitfield),
> since path_noexec() would get checked for S_ISREG. I can't come up with
> a rationale where one would want OPEN_MAYEXEC_ENFORCE_FILE but _not_
> OPEN_MAYEXEC_ENFORCE_MOUNT?

I don't see why it is an inclusion.

>
> (I can absolutely see wanting only OPEN_MAYEXEC_ENFORCE_MOUNT, or
> suddenly one has to go mark every loaded thing with the exec bit and
> most distros haven't done this to, for example, shared libraries. But
> setting the exec bit and then NOT wanting to enforce the mount check
> seems... not sensible?)
>
> Outside of this change, yes, I like this now -- it's much cleaner
> because we have all the checks in the same place where they belong. :)
>
>> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
>> index db1ce7af2563..5008a2566e79 100644
>> --- a/kernel/sysctl.c
>> +++ b/kernel/sysctl.c
>> @@ -113,6 +113,7 @@ static int sixty = 60;
>>
>> static int __maybe_unused neg_one = -1;
>> static int __maybe_unused two = 2;
>> +static int __maybe_unused three = 3;
>> static int __maybe_unused four = 4;
>> static unsigned long zero_ul;
>> static unsigned long one_ul = 1;
>
> Oh, are these still here? I thought they got removed (or at least made
> const). Where did that series go? Hmpf, see sysctl_vals, but yes, for
> now, this is fine.
>
>> @@ -888,7 +889,6 @@ static int proc_taint(struct ctl_table *table, int write,
>> return err;
>> }
>>
>> -#ifdef CONFIG_PRINTK
>> static int proc_dointvec_minmax_sysadmin(struct ctl_table *table, int write,
>> void *buffer, size_t *lenp, loff_t *ppos)
>> {
>> @@ -897,7 +897,6 @@ static int proc_dointvec_minmax_sysadmin(struct ctl_table *table, int write,
>>
>> return proc_dointvec_minmax(table, write, buffer, lenp, ppos);
>> }
>> -#endif
>>
>> /**
>> * struct do_proc_dointvec_minmax_conv_param - proc_dointvec_minmax() range checking structure
>> @@ -3264,6 +3263,15 @@ static struct ctl_table fs_table[] = {
>> .extra1 = SYSCTL_ZERO,
>> .extra2 = &two,
>> },
>> + {
>> + .procname = "open_mayexec_enforce",
>> + .data = &sysctl_open_mayexec_enforce,
>> + .maxlen = sizeof(int),
>> + .mode = 0600,
>> + .proc_handler = proc_dointvec_minmax_sysadmin,
>> + .extra1 = SYSCTL_ZERO,
>> + .extra2 = &three,
>> + },
>> #if defined(CONFIG_BINFMT_MISC) || defined(CONFIG_BINFMT_MISC_MODULE)
>> {
>> .procname = "binfmt_misc",
>> --
>> 2.27.0
>>
>