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seccomp: Make duplicate listener detection non-racy
commit dfe719f upstream. Currently, init_listener() tries to prevent adding a filter with SECCOMP_FILTER_FLAG_NEW_LISTENER if one of the existing filters already has a listener. However, this check happens without holding any lock that would prevent another thread from concurrently installing a new filter (potentially with a listener) on top of the ones we already have. Theoretically, this is also a data race: The plain load from current->seccomp.filter can race with concurrent writes to the same location. Fix it by moving the check into the region that holds the siglock to guard against concurrent TSYNC. (The "Fixes" tag points to the commit that introduced the theoretical data race; concurrent installation of another filter with TSYNC only became possible later, in commit 5189149 ("seccomp: allow TSYNC and USER_NOTIF together").) Fixes: 6a21cc5 ("seccomp: add a return code to trap to userspace") Reviewed-by: Tycho Andersen <tycho@tycho.pizza> Signed-off-by: Jann Horn <jannh@google.com> Signed-off-by: Kees Cook <keescook@chromium.org> Cc: stable@vger.kernel.org Link: https://lore.kernel.org/r/20201005014401.490175-1-jannh@google.com Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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