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SubjectRe: [PATCH] Capabilities: BUG when an invalid capability is requested
Quoting Eric Paris (eparis@redhat.com):
> If an invalid (large) capability is requested the capabilities system
> may panic as it is dereferencing an array of fixed (short) length. Its
> possible (and actually often happens) that the capability system
> accidentally stumbled into a valid memory region but it also regularly
> happens that it hits invalid memory and BUGs. If such an operation does
> get past cap_capable then the selinux system is sure to have problems as
> it already does a (simple) validity check and BUG. This is known to
> happen by the broken and buggy firegl driver.
>
> This patch cleanly checks all capable calls and BUG if a call is for an
> invalid capability. This will likely break the firegl driver for some
> situations, but it is the right thing to do. Garbage into a security
> system gets you killed/bugged
>
> Signed-off-by: Eric Paris <eparis@redhat.com>

I really don't like this, but I'm not sure we really have a choice.

Acked-by: Serge Hallyn <serue@us.ibm.com>

I suppose we can think later about whether it's worthwhile (a) having a
separate capable() function exported, keeping one without the check for
compiled-in use only, and/or (b) changing the cap_valid() definition to
be "(((unsigned int)cap) <= CAP_LAST_CAP)" which seems to work and shave
one whopping instruction. I suspect the answer will be no to both.

Thanks, Eric.

-serge

>
> ---
>
> kernel/capability.c | 5 +++++
> 1 files changed, 5 insertions(+), 0 deletions(-)
>
> diff --git a/kernel/capability.c b/kernel/capability.c
> index 33e51e7..50d9d99 100644
> --- a/kernel/capability.c
> +++ b/kernel/capability.c
> @@ -498,6 +498,11 @@ asmlinkage long sys_capset(cap_user_header_t header, const cap_user_data_t data)
> */
> int capable(int cap)
> {
> + if (unlikely(!cap_valid(cap))) {
> + printk(KERN_CRIT "capable() called with invalid cap=%u\n", cap);
> + BUG();
> + }
> +
> if (has_capability(current, cap)) {
> current->flags |= PF_SUPERPRIV;
> return 1;
>


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