[PATCH bpf v2 2/2] bpf: Avoid faultable build ID reads under mm locks

[Ihor Solodrai <ihor.solodrai@linux.dev>]

Sleepable build ID parsing can block in __kernel_read() [1], so the
stackmap sleepable path must not call it while holding mmap_lock or a
per-VMA read lock.

The issue and the fix are conceptually similar to a recent procfs
patch [2].

Resolve each covered VMA with a stable read-side reference, preferring
lock_vma_under_rcu() and falling back to mmap_read_lock() only long
enough to acquire the VMA read lock. Take a reference to the backing
file, drop the VMA lock, and then parse the build ID through
(sleepable) build_id_parse_file().

[1]: https://lore.kernel.org/all/20251218005818.614819-1-shakeel.butt@linux.dev/
[2]: https://lore.kernel.org/all/20260128183232.2854138-1-andrii@kernel.org/

Fixes: 777a8560fd29 ("lib/buildid: use __kernel_read() for sleepable context")
Assisted-by: Codex:gpt-5.4
Suggested-by: Puranjay Mohan <puranjay@kernel.org>
Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
---
 kernel/bpf/stackmap.c | 139 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 139 insertions(+)

diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
index 4ef0fd06cea5..de3d89e20a1e 100644
--- a/kernel/bpf/stackmap.c
+++ b/kernel/bpf/stackmap.c
@@ -9,6 +9,7 @@
 #include <linux/perf_event.h>
 #include <linux/btf_ids.h>
 #include <linux/buildid.h>
+#include <linux/mmap_lock.h>
 #include "percpu_freelist.h"
 #include "mmap_unlock_work.h"
 
@@ -158,6 +159,139 @@ static inline void stack_map_build_id_set_ip(struct bpf_stack_build_id *id)
 	memset(id->build_id, 0, BUILD_ID_SIZE_MAX);
 }
 
+enum stack_map_vma_lock_state {
+	STACK_MAP_LOCKED_NONE = 0,
+	STACK_MAP_LOCKED_VMA,
+	STACK_MAP_LOCKED_MMAP,
+};
+
+struct stack_map_vma_lock {
+	enum stack_map_vma_lock_state state;
+	struct vm_area_struct *vma;
+	struct mm_struct *mm;
+};
+
+static struct vm_area_struct *stack_map_lock_vma(struct stack_map_vma_lock *lock, unsigned long ip)
+{
+	struct mm_struct *mm = lock->mm;
+	struct vm_area_struct *vma;
+
+	if (WARN_ON_ONCE(!mm))
+		return NULL;
+
+	vma = lock_vma_under_rcu(mm, ip);
+	if (vma)
+		goto vma_locked;
+
+	if (!mmap_read_trylock(mm))
+		return NULL;
+
+	vma = vma_lookup(mm, ip);
+	if (!vma) {
+		mmap_read_unlock(mm);
+		return NULL;
+	}
+
+#ifdef CONFIG_PER_VMA_LOCK
+	if (!vma_start_read_locked(vma)) {
+		mmap_read_unlock(mm);
+		return NULL;
+	}
+	mmap_read_unlock(mm);
+#else
+	lock->state = STACK_MAP_LOCKED_MMAP;
+	lock->vma = vma;
+	return vma;
+#endif
+
+vma_locked:
+	lock->state = STACK_MAP_LOCKED_VMA;
+	lock->vma = vma;
+	return vma;
+}
+
+static void stack_map_unlock_vma(struct stack_map_vma_lock *lock)
+{
+	struct vm_area_struct *vma = lock->vma;
+	struct mm_struct *mm = lock->mm;
+
+	switch (lock->state) {
+	case STACK_MAP_LOCKED_VMA:
+		if (WARN_ON_ONCE(!vma))
+			break;
+		vma_end_read(vma);
+		break;
+	case STACK_MAP_LOCKED_MMAP:
+		if (WARN_ON_ONCE(!mm))
+			break;
+		mmap_read_unlock(mm);
+		break;
+	default:
+		break;
+	}
+
+	lock->state = STACK_MAP_LOCKED_NONE;
+	lock->vma = NULL;
+}
+
+static void stack_map_get_build_id_offset_sleepable(struct bpf_stack_build_id *id_offs,
+						    u32 trace_nr)
+{
+	struct mm_struct *mm = current->mm;
+	struct stack_map_vma_lock lock = {
+		.state = STACK_MAP_LOCKED_NONE,
+		.vma = NULL,
+		.mm = mm,
+	};
+	struct file *file, *prev_file = NULL;
+	unsigned long vm_pgoff, vm_start;
+	struct vm_area_struct *vma;
+	const char *prev_build_id;
+	u64 ip;
+
+	for (u32 i = 0; i < trace_nr; i++) {
+		ip = READ_ONCE(id_offs[i].ip);
+		vma = stack_map_lock_vma(&lock, ip);
+		if (!vma || !vma->vm_file) {
+			stack_map_build_id_set_ip(&id_offs[i]);
+			stack_map_unlock_vma(&lock);
+			continue;
+		}
+
+		file = vma->vm_file;
+		vm_pgoff = vma->vm_pgoff;
+		vm_start = vma->vm_start;
+
+		if (file == prev_file) {
+			memcpy(id_offs[i].build_id, prev_build_id, BUILD_ID_SIZE_MAX);
+			stack_map_unlock_vma(&lock);
+			goto build_id_valid;
+		}
+
+		file = get_file(file);
+		stack_map_unlock_vma(&lock);
+
+		/* build_id_parse_file() may block on filesystem reads */
+		if (build_id_parse_file(file, id_offs[i].build_id, NULL)) {
+			stack_map_build_id_set_ip(&id_offs[i]);
+			fput(file);
+			continue;
+		}
+
+		if (prev_file)
+			fput(prev_file);
+		prev_file = file;
+		prev_build_id = id_offs[i].build_id;
+
+build_id_valid:
+		id_offs[i].offset = (vm_pgoff << PAGE_SHIFT) + ip - vm_start;
+		id_offs[i].status = BPF_STACK_BUILD_ID_VALID;
+	}
+
+	if (prev_file)
+		fput(prev_file);
+}
+
 /*
  * Expects all id_offs[i].ip values to be set to correct initial IPs.
  * They will be subsequently:
@@ -178,6 +312,11 @@ static void stack_map_get_build_id_offset(struct bpf_stack_build_id *id_offs,
 	const char *prev_build_id;
 	int i;
 
+	if (may_fault && has_user_ctx) {
+		stack_map_get_build_id_offset_sleepable(id_offs, trace_nr);
+		return;
+	}
+
 	/* If the irq_work is in use, fall back to report ips. Same
 	 * fallback is used for kernel stack (!user) on a stackmap with
 	 * build_id.
-- 
2.53.0