From 0fb5751a6e411b1b466c06e50021818d9391df98 Mon Sep 17 00:00:00 2001 From: "Florent Revest (Anthropic)" Date: Sat, 26 Sep 2026 13:55:58 +0000 Subject: bpf: Wait for an RCU tasks grace period before freeing trampoline progs When a prog is detached from a trampoline, it is freed after an RCU grace period, or an RCU tasks trace one if it is sleepable. This covers the tasks that are running the prog, since the prog's enter helper takes the matching read lock before the prog is called. On a preemptible kernel, it doesn't cover a task that was preempted in the trampoline just before the enter helper. That task holds no lock yet, and it calls the prog after it was freed: BUG: KASAN: vmalloc-out-of-bounds in __bpf_prog_enter_recur+0x3a5/0x3f0 Read of size 8 at addr ffffc90000055040 by task candidate/110 CPU: 1 UID: 0 PID: 110 Comm: candidate Not tainted 7.3.0-rc2-00014-g15071f2a1263-dirty #2 PREEMPT(full) Call Trace: __bpf_prog_enter_recur+0x3a5/0x3f0 bpf_trampoline_6442509193+0x37/0xf1 __x64_sys_futex+0x9/0x410 do_syscall_64+0xb0/0x530 ... Wait for an RCU tasks grace period before the existing one when freeing a prog that was linked to a trampoline. An RCU tasks grace period only ends once the tasks that were preempted have run again, and bpf_tramp_image_put() already relies on it to free the image. It doesn't wait for tasks that sleep in the trampoline, the next commit takes care of those. Only progs that were linked to a trampoline can be called this way, so bpf_trampoline_add_prog() marks them and other progs are still freed as before. Fixes: e21aa341785c ("bpf: Fix fexit trampoline.") Reported-by: Junseo Lim Signed-off-by: Florent Revest (Anthropic) Signed-off-by: Alexei Starovoitov Link: https://patch.msgid.link/20260926135605.1217928-2-florent.revest@linux.dev Closes: https://lore.kernel.org/bpf/aqdrwVpanH3WGurX@omen-arch/ --- include/linux/bpf.h | 1 + 1 file changed, 1 insertion(+) (limited to 'include/linux') diff --git a/include/linux/bpf.h b/include/linux/bpf.h index 1d2676782d70..a8ec85127571 100644 --- a/include/linux/bpf.h +++ b/include/linux/bpf.h @@ -1770,6 +1770,7 @@ struct bpf_prog_aux { bool offload_requested; /* Program is bound and offloaded to the netdev. */ bool attach_btf_trace; /* true if attaching to BTF-enabled raw tp */ bool attach_tracing_prog; /* true if tracing another tracing program */ + bool tramp_linked; /* true if it was ever linked to a trampoline */ bool func_proto_unreliable; bool tail_call_reachable; bool xdp_has_frags; -- cgit v1.2.3 From 17637e1a581a22466ac3620a91e099683ff9cc6f Mon Sep 17 00:00:00 2001 From: "Florent Revest (Anthropic)" Date: Sat, 26 Sep 2026 13:55:59 +0000 Subject: bpf: Skip detached progs in trampoline images that are still in use bpf_tramp_image_put() makes sure a trampoline image is not freed while a task may still be running in it, but nothing similar is done for the progs called by that image. Detach drops the last prog reference right away and the prog is freed after grace periods, on the basis that a task still in the traced function skips the fexit progs once the nop at ip_after_call is patched to a jump. That leaves out a task sleeping in a sleepable prog that runs before the detached one, which no grace period waits for: CPU 0 CPU 1 in image I, sleeping in prog S detach P from I's trampoline -> new image, bpf_tramp_image_put(I) bpf_prog_put(P), last ref grace periods, P freed back from S __bpf_prog_enter(P) call P->bpf_func If S and P are fexit progs the task is already past the patched jump, and fentry only images don't have one. On x86 this is an int3 in poisoned bpf_prog_pack memory: Oops: int3: 0000 [#1] SMP NOPTI CPU: 18 UID: 0 PID: 94573 Comm: x169 Not tainted 6.18.44 #1 PREEMPT(lazy) RIP: 0010:0xffffffffc0601d8d Call Trace: ? bpf_trampoline_6442515411+0x1a4/0x21b bpf_lsm_bprm_committed_creds+0x5/0x10 security_bprm_committed_creds+0x5f/0x70 begin_new_exec+0x2d6/0x410 ... We hit this in production when progs attached through trampolines got detached while their hooks were busy, and it was independently found with a fuzzer and KASAN. Have the JITs emit a patchable nop in front of each prog call sequence and record it in the image. When a prog is detached, patch its nop to a jump over the call sequence. Progs that stay attached keep running for the tasks that are in the image, and ip_after_call isn't needed anymore. The task can be in any image that isn't freed yet, not only in the current one. It sleeps in image I1 that calls S, P and Q, then P is detached and the trampoline moves to image I2, then Q is detached and its call is still in I1. So the trampoline keeps a list of its images until they are freed, and detaching a prog patches its nop in all of them. Images hold a reference on the trampoline for that long. On riscv and loongarch a jump of any range takes several instructions, and a task preempted in the middle of them could resume into half of the new sequence. The nop is a single instruction there, patched to a near branch through arch_bpf_trampoline_skip(). With the extra nops, BPF_MAX_TRAMP_LINKS progs no longer fit in a page on x86 and arm64 (and already didn't on powerpc), so lower the limit there like s390 does. Fixes: e21aa341785c ("bpf: Fix fexit trampoline.") Reported-by: Sechang Lim Suggested-by: Alexei Starovoitov Signed-off-by: Florent Revest (Anthropic) Signed-off-by: Alexei Starovoitov Link: https://patch.msgid.link/20260926135605.1217928-3-florent.revest@linux.dev Closes: https://lore.kernel.org/bpf/20260815071927.147049-1-zirajs7@gmail.com/ --- include/linux/bpf.h | 44 ++++++++++++++++++++++++++++++++++++++++---- 1 file changed, 40 insertions(+), 4 deletions(-) (limited to 'include/linux') diff --git a/include/linux/bpf.h b/include/linux/bpf.h index a8ec85127571..0ecb9418dfbd 100644 --- a/include/linux/bpf.h +++ b/include/linux/bpf.h @@ -1258,11 +1258,15 @@ struct btf_func_model { #define BPF_TRAMP_F_INDIRECT BIT(8) /* Each call __bpf_prog_enter + call bpf_func + call __bpf_prog_exit is ~50 - * bytes on x86. + * bytes on x86. The trampoline image has to fit in PAGE_SIZE. */ enum { -#if defined(__s390x__) +#if defined(__s390x__) || defined(__powerpc64__) BPF_MAX_TRAMP_LINKS = 27, +#elif defined(__x86_64__) + BPF_MAX_TRAMP_LINKS = 36, +#elif defined(__aarch64__) + BPF_MAX_TRAMP_LINKS = 37, #else BPF_MAX_TRAMP_LINKS = 38, #endif @@ -1315,6 +1319,7 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i void *arch_alloc_bpf_trampoline(unsigned int size); void arch_free_bpf_trampoline(void *image, unsigned int size); int __must_check arch_protect_bpf_trampoline(void *image, unsigned int size); +int arch_bpf_trampoline_skip(void *nop, void *target); int arch_bpf_trampoline_size(const struct btf_func_model *m, u32 flags, struct bpf_tramp_nodes *tnodes, void *func_addr); @@ -1363,19 +1368,48 @@ enum bpf_tramp_prog_type { BPF_TRAMP_FSESSION, }; +/* + * Each prog call in a trampoline image is preceded by a nop. When the prog is + * detached, the nop is patched to a jump to target, right after the call, so + * that tasks still running in the image skip the prog. + */ +struct bpf_tramp_skip { + struct bpf_prog *prog; + void *nop; + void *target; +}; + struct bpf_tramp_image { void *image; int size; struct bpf_ksym ksym; struct percpu_ref pcref; - void *ip_after_call; - void *ip_epilogue; + bool call_orig; + /* entry in tr->images, the image holds a reference on tr */ + struct bpf_trampoline *tr; + struct list_head list; + int nr_skips; + struct bpf_tramp_skip *skips; union { struct rcu_head rcu; struct work_struct work; }; }; +static inline void bpf_tramp_image_add_skip(struct bpf_tramp_image *im, struct bpf_prog *prog, + void *nop, void *target) +{ + struct bpf_tramp_skip *skip; + + /* struct_ops trampolines and arch_bpf_trampoline_size() have no image */ + if (!im || !im->skips) + return; + skip = &im->skips[im->nr_skips++]; + skip->prog = prog; + skip->nop = nop; + skip->target = target; +} + struct bpf_trampoline { /* hlist for trampoline_key_table */ struct hlist_node hlist_key; @@ -1402,6 +1436,8 @@ struct bpf_trampoline { int progs_cnt[BPF_TRAMP_MAX]; /* Executable image of trampoline */ struct bpf_tramp_image *cur_image; + /* Images not freed yet, cur_image and older ones still in use */ + struct list_head images; /* Used as temporary old image storage for multi_attach */ struct { struct bpf_tramp_image *old_image; -- cgit v1.2.3