From: Ihor Solodrai Date: Mon, 25 May 2026 22:39:47 +0000 (-0700) Subject: bpf: Avoid faultable build ID reads under mm locks X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=fad3021faf7b0b64e9daea41c5662b65c8ad7379;p=thirdparty%2Fkernel%2Flinux.git bpf: Avoid faultable build ID reads under mm locks 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]. A similar VMA locking pattern has already been used in PROCMAP_QUERY [3]. Resolve each covered VMA with a stable read-side reference, preferring lock_vma_under_rcu() and falling back to mmap_read_trylock() 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(). We have to use mmap_read_trylock() (and give up on failure) in this context because taking mmap_read_lock() is generally unsafe on code paths reachable from BPF programs [4], and may lead to deadlocks. [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/ [3] https://lore.kernel.org/all/20250808152850.2580887-1-surenb@google.com/ [4] https://lore.kernel.org/bpf/2895ecd8-df1e-4cc0-b9f9-aef893dc2360@linux.dev/ Fixes: d4dd9775ec24 ("bpf: wire up sleepable bpf_get_stack() and bpf_get_task_stack() helpers") Suggested-by: Puranjay Mohan Signed-off-by: Ihor Solodrai Signed-off-by: Andrii Nakryiko Link: https://lore.kernel.org/bpf/20260525223948.1920986-3-ihor.solodrai@linux.dev --- diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c index e23be7d445038..c53cfd9a67cfa 100644 --- a/kernel/bpf/stackmap.c +++ b/kernel/bpf/stackmap.c @@ -9,6 +9,7 @@ #include #include #include +#include #include "percpu_freelist.h" #include "mmap_unlock_work.h" @@ -174,6 +175,109 @@ static inline void stack_map_build_id_set_valid(struct bpf_stack_build_id *id, memcpy(id->build_id, build_id, BUILD_ID_SIZE_MAX); } +struct stack_map_vma_lock { + struct vm_area_struct *vma; + struct mm_struct *mm; +}; + +/* + * Acquire a stable read-side reference on the VMA covering @ip. + * + * With CONFIG_PER_VMA_LOCK=y this returns a VMA with its per-VMA read + * lock held and mmap_lock dropped, so the caller may sleep. + * + * With CONFIG_PER_VMA_LOCK=n it returns a VMA with mmap_lock still + * held; the caller must snapshot any fields it needs and pin vm_file + * with get_file() before stack_map_unlock_vma() drops mmap_lock, as + * the VMA may be split, merged, or freed after that. + * + * Returns NULL on failure, in which case no lock is held. + */ +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; + + /* noop under !CONFIG_PER_VMA_LOCK */ + vma = lock_vma_under_rcu(mm, ip); + if (vma) { + lock->vma = vma; + return vma; + } + + /* + * Taking mmap_read_lock() is unsafe here, because the caller BPF + * program might already hold it, causing a deadlock. + */ + 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); +#endif + + lock->vma = vma; + return vma; +} + +static void stack_map_unlock_vma(struct stack_map_vma_lock *lock) +{ +#ifdef CONFIG_PER_VMA_LOCK + vma_end_read(lock->vma); +#else + mmap_read_unlock(lock->mm); +#endif + 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 = { .mm = mm }; + struct vm_area_struct *vma; + struct file *file; + u64 offset; + 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) { + stack_map_build_id_set_ip(&id_offs[i]); + continue; + } + if (vma_is_anonymous(vma) || !vma->vm_file) { + stack_map_build_id_set_ip(&id_offs[i]); + stack_map_unlock_vma(&lock); + continue; + } + + file = get_file(vma->vm_file); + offset = stack_map_build_id_offset(vma->vm_pgoff, vma->vm_start, ip); + 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]); + else + stack_map_build_id_set_valid(&id_offs[i], offset, id_offs[i].build_id); + fput(file); + } +} + /* * Expects all id_offs[i].ip values to be set to correct initial IPs. * They will be subsequently: @@ -194,6 +298,11 @@ static void stack_map_get_build_id_offset(struct bpf_stack_build_id *id_offs, const unsigned char *prev_build_id = NULL; 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.