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    SubjectRe: [PATCH v2] fs: select: fix information leak to userspace
    On Sun, Nov 14, 2010 at 18:06 -0800, Andrew Morton wrote:
    > On Sun, 14 Nov 2010 12:25:33 +0300 Vasiliy Kulikov <segoon@openwall.com> wrote:
    >
    > > On some architectures __kernel_suseconds_t is int. On these archs
    > > struct timeval has padding bytes at the end. This struct is copied to
    > > userspace with these padding bytes uninitialized. This leads to leaking
    > > of contents of kernel stack memory.
    > >
    > > Signed-off-by: Vasiliy Kulikov <segoon@openwall.com>
    > > ---
    > > Patch v1 used memset(), it was waste of cycles on almost all archs.
    > >
    > > Compile tested.
    > >
    > > fs/select.c | 7 ++++---
    > > 1 files changed, 4 insertions(+), 3 deletions(-)
    > >
    > > diff --git a/fs/select.c b/fs/select.c
    > > index b7b10aa..43d4805 100644
    > > --- a/fs/select.c
    > > +++ b/fs/select.c
    > > @@ -288,7 +288,6 @@ static int poll_select_copy_remaining(struct timespec *end_time, void __user *p,
    > > int timeval, int ret)
    > > {
    > > struct timespec rts;
    > > - struct timeval rtv;
    > >
    > > if (!p)
    > > return ret;
    > > @@ -306,8 +305,10 @@ static int poll_select_copy_remaining(struct timespec *end_time, void __user *p,
    > > rts.tv_sec = rts.tv_nsec = 0;
    > >
    > > if (timeval) {
    > > - rtv.tv_sec = rts.tv_sec;
    > > - rtv.tv_usec = rts.tv_nsec / NSEC_PER_USEC;
    > > + struct timeval rtv = {
    > > + .tv_sec = rts.tv_sec,
    > > + .tv_usec = rts.tv_nsec / NSEC_PER_USEC
    > > + };
    > >
    > > if (!copy_to_user(p, &rtv, sizeof(rtv)))
    > > return ret;
    >
    > Please check the assembly code - this will still leave four bytes of
    > uninitalised stack data in 'rtv', surely.

    This concrete c code generates movl + movq, movl would zero unnamed
    4 bytes. However, I cannot find whether this behavior is guaranteed...

    --
    Vasiliy Kulikov


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