Roman Gushchin roman.gushchin@linux.dev writes:
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.
Not true.
It added a limit on the number of siginfo structures that a container may allocate. Have you tried not limiting your container?
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.
Note. There are always conditions when the allocation may fail. The structure is allocated with __GFP_ATOMIC so it is much more likely to fail than a typical kernel memory allocation.
But I agree it does look like there is a quality of implementation issue here.
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.
Instead please just give the container and unlimited number of siginfo structures it can play with.
The maximum for rlimit(RLIM_SIGPENDING) is the rlimit(RLIM_SIGPENDING) value when the user namespace is created.
Given that it took 3 and half years to report this. I am going to say this really looks like a userspace bug.
Beyond that your patch is actually buggy, and should not be applied.
If we want to change the semantics and ignore the maximum number of pending signals in a container (when override_rlimit is set) then the code should change the computation of the max value (pegging it at LONG_MAX) and not ignore it.
As it is the patch below disables the check that keeps the ucount counters from wrapping around. That makes it possible for someone to overflow those counters and get into all kinds of trouble.
Eric
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);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;