[PATCH] capabilities: fix undefined behavior in bit shift for CAP_TO_MASK

[Serge E. Hallyn]

On Mon, Oct 31, 2022 at 07:18:54AM -0700, Andrew G. Morgan wrote:
> Acked-by: Andrew G. Morgan <morgan at kernel.org>
> 
> 
> On Mon, Oct 31, 2022 at 4:25 AM Gaosheng Cui <cuigaosheng1 at huawei.com> wrote:
> >
> > Shifting signed 32-bit value by 31 bits is undefined, so changing
> > significant bit to unsigned. The UBSAN warning calltrace like below:
> >
> > UBSAN: shift-out-of-bounds in security/commoncap.c:1252:2
> > left shift of 1 by 31 places cannot be represented in type 'int'
> > Call Trace:
> >  <TASK>
> >  dump_stack_lvl+0x7d/0xa5
> >  dump_stack+0x15/0x1b
> >  ubsan_epilogue+0xe/0x4e
> >  __ubsan_handle_shift_out_of_bounds+0x1e7/0x20c
> >  cap_task_prctl+0x561/0x6f0
> >  security_task_prctl+0x5a/0xb0
> >  __x64_sys_prctl+0x61/0x8f0
> >  do_syscall_64+0x58/0x80
> >  entry_SYSCALL_64_after_hwframe+0x63/0xcd
> >  </TASK>
> >
> > Fixes: e338d263a76a ("Add 64-bit capability support to the kernel")
> > Signed-off-by: Gaosheng Cui <cuigaosheng1 at huawei.com>

Reviewed-by: Serge Hallyn <serge at hallyn.com>

Paul, do you mind including this in your lsm tree?

thanks,
-serge

> > ---
> >  include/uapi/linux/capability.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/include/uapi/linux/capability.h b/include/uapi/linux/capability.h
> > index 463d1ba2232a..3d61a0ae055d 100644
> > --- a/include/uapi/linux/capability.h
> > +++ b/include/uapi/linux/capability.h
> > @@ -426,7 +426,7 @@ struct vfs_ns_cap_data {
> >   */
> >
> >  #define CAP_TO_INDEX(x)     ((x) >> 5)        /* 1 << 5 == bits in __u32 */
> > -#define CAP_TO_MASK(x)      (1 << ((x) & 31)) /* mask for indexed __u32 */
> > +#define CAP_TO_MASK(x)      (1U << ((x) & 31)) /* mask for indexed __u32 */
> >
> >
> >  #endif /* _UAPI_LINUX_CAPABILITY_H */
> > --
> > 2.25.1
> >