[PATCH bpf] bpf: Avoid deadlock when using queue and stack maps from NMI

[PATCH bpf] bpf: Avoid deadlock when using queue and stack maps from NMI

[Toke Høiland-Jørgensen]

Sysbot discovered that the queue and stack maps can deadlock if they are
being used from a BPF program that can be called from NMI context (such as
one that is attached to a perf HW counter event). To fix this, add an
in_nmi() check and use raw_spin_trylock() in NMI context, erroring out if
grabbing the lock fails.

Fixes: f1a2e44a3aec ("bpf: add queue and stack maps")
Reported-by: Hsin-Wei Hung <hsinweih@uci.edu>
Tested-by: Hsin-Wei Hung <hsinweih@uci.edu>
Co-developed-by: Hsin-Wei Hung <hsinweih@uci.edu>
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
---
 kernel/bpf/queue_stack_maps.c | 21 ++++++++++++++++++---
 1 file changed, 18 insertions(+), 3 deletions(-)

diff --git a/kernel/bpf/queue_stack_maps.c b/kernel/bpf/queue_stack_maps.c
index 8d2ddcb7566b..d869f51ea93a 100644
--- a/kernel/bpf/queue_stack_maps.c
+++ b/kernel/bpf/queue_stack_maps.c
@@ -98,7 +98,12 @@ static long __queue_map_get(struct bpf_map *map, void *value, bool delete)
 	int err = 0;
 	void *ptr;
 
-	raw_spin_lock_irqsave(&qs->lock, flags);
+	if (in_nmi()) {
+		if (!raw_spin_trylock_irqsave(&qs->lock, flags))
+			return -EBUSY;
+	} else {
+		raw_spin_lock_irqsave(&qs->lock, flags);
+	}
 
 	if (queue_stack_map_is_empty(qs)) {
 		memset(value, 0, qs->map.value_size);
@@ -128,7 +133,12 @@ static long __stack_map_get(struct bpf_map *map, void *value, bool delete)
 	void *ptr;
 	u32 index;
 
-	raw_spin_lock_irqsave(&qs->lock, flags);
+	if (in_nmi()) {
+		if (!raw_spin_trylock_irqsave(&qs->lock, flags))
+			return -EBUSY;
+	} else {
+		raw_spin_lock_irqsave(&qs->lock, flags);
+	}
 
 	if (queue_stack_map_is_empty(qs)) {
 		memset(value, 0, qs->map.value_size);
@@ -193,7 +203,12 @@ static long queue_stack_map_push_elem(struct bpf_map *map, void *value,
 	if (flags & BPF_NOEXIST || flags > BPF_EXIST)
 		return -EINVAL;
 
-	raw_spin_lock_irqsave(&qs->lock, irq_flags);
+	if (in_nmi()) {
+		if (!raw_spin_trylock_irqsave(&qs->lock, irq_flags))
+			return -EBUSY;
+	} else {
+		raw_spin_lock_irqsave(&qs->lock, irq_flags);
+	}
 
 	if (queue_stack_map_is_full(qs)) {
 		if (!replace) {
-- 
2.42.0

Re: [PATCH bpf] bpf: Avoid deadlock when using queue and stack maps from NMI

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to bpf/bpf.git (master)
by Alexei Starovoitov <ast@kernel.org>:

On Mon, 11 Sep 2023 15:28:14 +0200 you wrote:
> Sysbot discovered that the queue and stack maps can deadlock if they are
> being used from a BPF program that can be called from NMI context (such as
> one that is attached to a perf HW counter event). To fix this, add an
> in_nmi() check and use raw_spin_trylock() in NMI context, erroring out if
> grabbing the lock fails.
> 
> Fixes: f1a2e44a3aec ("bpf: add queue and stack maps")
> Reported-by: Hsin-Wei Hung <hsinweih@uci.edu>
> Tested-by: Hsin-Wei Hung <hsinweih@uci.edu>
> Co-developed-by: Hsin-Wei Hung <hsinweih@uci.edu>
> Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
> 
> [...]

Here is the summary with links:
  - [bpf] bpf: Avoid deadlock when using queue and stack maps from NMI
    https://git.kernel.org/bpf/bpf/c/a34a9f1a19af

You are awesome, thank you!
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