[PATCH bpf v6 01/10] bpf: Make post-verification instruction rewrites killable

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

After do_check() returns, the verifier runs several instruction rewrite
passes. Some of them patch or remove one instruction at a time. Each
operation moves the remaining instruction and auxiliary-data arrays and
adjusts all branch offsets, making the overall work quadratic in the
program length.

A privileged loader can submit 131072 unconditional jumps by zero followed
by a valid return. Verification finishes quickly, but bpf_opt_remove_nops()
then spends a long time removing each jump separately. Since this
post-verification work neither checks for signals nor reschedules, a pending
SIGKILL cannot terminate the task until the rewrite finishes.

Make bpf_patch_insn_data() and verifier_remove_insns() common cancellation
and rescheduling points. These helpers run from BPF_PROG_LOAD process
context, and bpf_patch_insn_data() can already sleep while reallocating
auxiliary data.

Report interrupted constant blinding as -EINTR and propagate it through
both JIT paths, including kernels that permit interpreter fallback.
Other blinding failures retain the existing fallback behavior.

This does not reduce the quadratic cost of the rewrite passes, but it makes
the work preemptible and allows a killed loader to be torn down promptly.

Fixes: 52875a04f4b2 ("bpf: verifier: remove dead code")
Reported-by: Nicholas Carlini <npc@anthropic.com>
Suggested-by: Nicholas Carlini <npc@anthropic.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 kernel/bpf/core.c   | 21 +++++++++++++++++----
 kernel/bpf/fixups.c | 24 ++++++++++++++++++++++--
 2 files changed, 39 insertions(+), 6 deletions(-)

diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index 8b294dfc1ad4..2e3bf8113ae9 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -19,6 +19,7 @@
 
 #include <uapi/linux/btf.h>
 #include <linux/filter.h>
+#include <linux/sched/signal.h>
 #include <linux/skbuff.h>
 #include <linux/static_call.h>
 #include <linux/vmalloc.h>
@@ -1619,6 +1620,8 @@ struct bpf_prog *bpf_jit_blind_constants(struct bpf_verifier_env *env, struct bp
 			 * fix it up here on error.
 			 */
 			bpf_jit_prog_release_other(prog, clone);
+			if (env && fatal_signal_pending(current))
+				return ERR_PTR(-EINTR);
 			return IS_ERR(tmp) ? tmp : ERR_PTR(-ENOMEM);
 		}
 
@@ -2636,11 +2639,14 @@ static struct bpf_prog *bpf_prog_jit_compile(struct bpf_verifier_env *env, struc
 	orig_prog = prog;
 	prog = bpf_jit_blind_constants(env, prog);
 	/*
-	 * If blinding was requested and we failed during blinding, we must fall
-	 * back to the interpreter.
+	 * Fall back to the interpreter after blinding failures, except when
+	 * the loader was killed.
 	 */
-	if (IS_ERR(prog))
+	if (IS_ERR(prog)) {
+		if (PTR_ERR(prog) == -EINTR)
+			return prog;
 		goto out_restore;
+	}
 
 	prog = bpf_int_jit_compile(env, prog);
 	if (prog->jited) {
@@ -2659,6 +2665,8 @@ static struct bpf_prog *bpf_prog_jit_compile(struct bpf_verifier_env *env, struc
 struct bpf_prog *__bpf_prog_select_runtime(struct bpf_verifier_env *env, struct bpf_prog *fp,
 					   int *err)
 {
+	struct bpf_prog *jit_prog;
+
 	/* In case of BPF to BPF calls, verifier did all the prep
 	 * work with regards to JITing, etc.
 	 */
@@ -2681,7 +2689,12 @@ struct bpf_prog *__bpf_prog_select_runtime(struct bpf_verifier_env *env, struct
 		if (*err)
 			return fp;
 
-		fp = bpf_prog_jit_compile(env, fp);
+		jit_prog = bpf_prog_jit_compile(env, fp);
+		if (IS_ERR(jit_prog)) {
+			*err = PTR_ERR(jit_prog);
+			return fp;
+		}
+		fp = jit_prog;
 		bpf_prog_jit_attempt_done(fp);
 		if (!fp->jited && jit_needed) {
 			*err = -ENOTSUPP;
diff --git a/kernel/bpf/fixups.c b/kernel/bpf/fixups.c
index 52d3cec33672..d6f83521fc78 100644
--- a/kernel/bpf/fixups.c
+++ b/kernel/bpf/fixups.c
@@ -8,6 +8,7 @@
 #include <linux/bsearch.h>
 #include <linux/sort.h>
 #include <linux/perf_event.h>
+#include <linux/sched/signal.h>
 #include <net/xdp.h>
 #include "disasm.h"
 
@@ -306,12 +307,28 @@ static void adjust_poke_descs(struct bpf_prog *prog, u32 off, u32 len)
 	}
 }
 
+/*
+ * Some post-verification instruction rewriting passes require an
+ * O(prog->len) operation per instruction. Keep their shared primitives
+ * killable and preemptible.
+ */
+static bool bpf_rewrite_must_abort(void)
+{
+	if (fatal_signal_pending(current))
+		return true;
+	cond_resched();
+	return false;
+}
+
 struct bpf_prog *bpf_patch_insn_data(struct bpf_verifier_env *env, u32 off,
 				     const struct bpf_insn *patch, u32 len)
 {
 	struct bpf_prog *new_prog;
 	struct bpf_insn_aux_data *new_data = NULL;
 
+	if (bpf_rewrite_must_abort())
+		return NULL;
+
 	if (len > 1) {
 		new_data = vrealloc(env->insn_aux_data,
 				    array_size(env->prog->len + len - 1,
@@ -523,6 +540,9 @@ static int verifier_remove_insns(struct bpf_verifier_env *env, u32 off, u32 cnt)
 	unsigned int orig_prog_len = env->prog->len;
 	int err;
 
+	if (bpf_rewrite_must_abort())
+		return -EINTR;
+
 	if (bpf_prog_is_offloaded(env->prog->aux))
 		bpf_prog_offload_remove_insns(env, off, cnt);
 
@@ -1356,7 +1376,7 @@ int bpf_jit_subprogs(struct bpf_verifier_env *env)
 		}
 		prog = bpf_jit_blind_constants(env, prog);
 		if (IS_ERR(prog)) {
-			err = -ENOMEM;
+			err = PTR_ERR(prog);
 			prog = orig_prog;
 			goto out_restore;
 		}
@@ -1433,7 +1453,7 @@ int bpf_fixup_call_args(struct bpf_verifier_env *env)
 		err = bpf_jit_subprogs(env);
 		if (err == 0)
 			return 0;
-		if (err == -EFAULT)
+		if (err == -EFAULT || err == -EINTR)
 			return err;
 	}
 #ifndef CONFIG_BPF_JIT_ALWAYS_ON
-- 
2.53.0