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    SubjectRe: [tip:sched/core] sched/core: Fix a potential double-fetch bug in sched_copy_attr()

    * Thomas Gleixner <tglx@linutronix.de> wrote:

    > On Mon, 21 Jan 2019, tip-bot for Kangjie Lu wrote:
    > > Commit-ID: 120e4e76857ddbc9268e1aa3f9de61a498e84618
    > > Gitweb: https://git.kernel.org/tip/120e4e76857ddbc9268e1aa3f9de61a498e84618
    > > Author: Kangjie Lu <kjlu@umn.edu>
    > > AuthorDate: Wed, 9 Jan 2019 01:45:24 -0600
    > > Committer: Ingo Molnar <mingo@kernel.org>
    > > CommitDate: Mon, 21 Jan 2019 11:26:17 +0100
    > >
    > > sched/core: Fix a potential double-fetch bug in sched_copy_attr()
    > >
    > > "uattr->size" is copied in from user space and checked. However, it is
    > > copied in again after the security check. A malicious user may race to
    > > change it. The fix sets uattr->size to be the checked size.
    > >
    > > Signed-off-by: Kangjie Lu <kjlu@umn.edu>
    > > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    > > Cc: Linus Torvalds <torvalds@linux-foundation.org>
    > > Cc: Peter Zijlstra <peterz@infradead.org>
    > > Cc: Thomas Gleixner <tglx@linutronix.de>
    > > Cc: pakki001@umn.edu
    > > Cc: <stable@vger.kernel.org>
    > > Link: https://lkml.kernel.org/r/20190109074524.10176-1-kjlu@umn.edu
    > > Signed-off-by: Ingo Molnar <mingo@kernel.org>
    > > ---
    > > kernel/sched/core.c | 3 +++
    > > 1 file changed, 3 insertions(+)
    > >
    > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
    > > index a674c7db2f29..d4d3514c4fe9 100644
    > > --- a/kernel/sched/core.c
    > > +++ b/kernel/sched/core.c
    > > @@ -4499,6 +4499,9 @@ static int sched_copy_attr(struct sched_attr __user *uattr, struct sched_attr *a
    > > if (ret)
    > > return -EFAULT;
    > >
    > > + /* In case attr->size was changed by user-space: */
    > > + attr->size = size;
    > > +
    >
    > Just when pondering to send that to Linus, I tried to write up a concise
    > summary for this which made me look at the patch.
    >
    > If the size changed, then its clear that user space fiddled with the date
    > between the size fetch and the full copy from user. So why restoring the
    > size instead of doing the obvious:
    >
    > if (attr->size != size)
    > return -ECRAP;
    >
    > Hmm?

    Yeah, indeed - and that's a much more reliable interface behavior in any
    case. It's probably also faster.

    Thanks,

    Ingo

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