Re: [PATCH] signal: restore the override_rlimit logic

[Alexey Gladkov <legion@kernel.org>]

On Thu, Oct 31, 2024 at 08:04:38PM +0000, Roman Gushchin wrote:
> Prior to commit d64696905554 ("Reimplement RLIMIT_SIGPENDING on top of
> ucounts") UCOUNT_RLIMIT_SIGPENDING rlimit was not enforced for a class
> of signals. However now it's enforced unconditionally, even if
> override_rlimit is set. This behavior change caused production issues.
> 
> For example, if the limit is reached and a process receives a SIGSEGV
> signal, sigqueue_alloc fails to allocate the necessary resources for the
> signal delivery, preventing the signal from being delivered with
> siginfo. This prevents the process from correctly identifying the fault
> address and handling the error. From the user-space perspective,
> applications are unaware that the limit has been reached and that the
> siginfo is effectively 'corrupted'. This can lead to unpredictable
> behavior and crashes, as we observed with java applications.
> 
> Fix this by passing override_rlimit into inc_rlimit_get_ucounts() and
> skip the comparison to max there if override_rlimit is set. This
> effectively restores the old behavior.
> 
> Fixes: d64696905554 ("Reimplement RLIMIT_SIGPENDING on top of ucounts")
> Signed-off-by: Roman Gushchin <roman.gushchin@linux.dev>
> Co-developed-by: Andrei Vagin <avagin@google.com>
> Signed-off-by: Andrei Vagin <avagin@google.com>
> Cc: Kees Cook <kees@kernel.org>
> Cc: "Eric W. Biederman" <ebiederm@xmission.com>
> Cc: Alexey Gladkov <legion@kernel.org>
> Cc: <stable@vger.kernel.org>
> ---
>  include/linux/user_namespace.h | 3 ++-
>  kernel/signal.c                | 3 ++-
>  kernel/ucount.c                | 5 +++--
>  3 files changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
> index 3625096d5f85..7183e5aca282 100644
> --- a/include/linux/user_namespace.h
> +++ b/include/linux/user_namespace.h
> @@ -141,7 +141,8 @@ static inline long get_rlimit_value(struct ucounts *ucounts, enum rlimit_type ty
>  
>  long inc_rlimit_ucounts(struct ucounts *ucounts, enum rlimit_type type, long v);
>  bool dec_rlimit_ucounts(struct ucounts *ucounts, enum rlimit_type type, long v);
> -long inc_rlimit_get_ucounts(struct ucounts *ucounts, enum rlimit_type type);
> +long inc_rlimit_get_ucounts(struct ucounts *ucounts, enum rlimit_type type,
> +			    bool override_rlimit);
>  void dec_rlimit_put_ucounts(struct ucounts *ucounts, enum rlimit_type type);
>  bool is_rlimit_overlimit(struct ucounts *ucounts, enum rlimit_type type, unsigned long max);
>  
> diff --git a/kernel/signal.c b/kernel/signal.c
> index 4344860ffcac..cbabb2d05e0a 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -419,7 +419,8 @@ __sigqueue_alloc(int sig, struct task_struct *t, gfp_t gfp_flags,
>  	 */
>  	rcu_read_lock();
>  	ucounts = task_ucounts(t);
> -	sigpending = inc_rlimit_get_ucounts(ucounts, UCOUNT_RLIMIT_SIGPENDING);
> +	sigpending = inc_rlimit_get_ucounts(ucounts, UCOUNT_RLIMIT_SIGPENDING,
> +					    override_rlimit);
>  	rcu_read_unlock();
>  	if (!sigpending)
>  		return NULL;
> diff --git a/kernel/ucount.c b/kernel/ucount.c
> index 16c0ea1cb432..046b3d57ebb4 100644
> --- a/kernel/ucount.c
> +++ b/kernel/ucount.c
> @@ -307,7 +307,8 @@ void dec_rlimit_put_ucounts(struct ucounts *ucounts, enum rlimit_type type)
>  	do_dec_rlimit_put_ucounts(ucounts, NULL, type);
>  }
>  
> -long inc_rlimit_get_ucounts(struct ucounts *ucounts, enum rlimit_type type)
> +long inc_rlimit_get_ucounts(struct ucounts *ucounts, enum rlimit_type type,
> +			    bool override_rlimit)
>  {
>  	/* Caller must hold a reference to ucounts */
>  	struct ucounts *iter;
> @@ -316,7 +317,7 @@ long inc_rlimit_get_ucounts(struct ucounts *ucounts, enum rlimit_type type)
>  
>  	for (iter = ucounts; iter; iter = iter->ns->ucounts) {
>  		long new = atomic_long_add_return(1, &iter->rlimit[type]);
> -		if (new < 0 || new > max)
> +		if (new < 0 || (!override_rlimit && (new > max)))
>  			goto unwind;
>  		if (iter == ucounts)
>  			ret = new;

It's a bad patch. If we do as you suggest, it will
do_dec_rlimit_put_ucounts() in case of overflow. This means you'll
break the counter and there will be an extra decrement in __sigqueue_free().
We can't just ignore the overflow here.

-- 
Rgrds, legion