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Message-ID: <ZrIj9jkXqpKXRuS7@krava>
Date: Tue, 6 Aug 2024 15:24:06 +0200
From: Jiri Olsa <olsajiri@...il.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Jiri Olsa <olsajiri@...il.com>, Juri Lelli <juri.lelli@...hat.com>,
bpf <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
Artem Savkov <asavkov@...hat.com>
Subject: Re: NULL pointer deref when running BPF monitor program (6.11.0-rc1)
On Mon, Aug 05, 2024 at 10:00:40AM -0700, Alexei Starovoitov wrote:
> On Mon, Aug 5, 2024 at 9:50 AM Jiri Olsa <olsajiri@...il.com> wrote:
> >
> > On Mon, Aug 05, 2024 at 11:20:11AM +0200, Juri Lelli wrote:
> >
> > SNIP
> >
> > > [ 154.566882] BUG: kernel NULL pointer dereference, address: 000000000000040c
> > > [ 154.573844] #PF: supervisor read access in kernel mode
> > > [ 154.578982] #PF: error_code(0x0000) - not-present page
> > > [ 154.584122] PGD 146fff067 P4D 146fff067 PUD 10fc00067 PMD 0
> > > [ 154.589780] Oops: Oops: 0000 [#1] PREEMPT SMP NOPTI
> > > [ 154.594659] CPU: 28 UID: 0 PID: 2234 Comm: thread0-13 Kdump: loaded Not tainted 6.11.0-rc1 #8
> > > [ 154.603179] Hardware name: Dell Inc. PowerEdge R740/04FC42, BIOS 2.10.2 02/24/2021
> > > [ 154.610744] RIP: 0010:bpf_prog_ec8173ca2868eb50_handle__sched_pi_setprio+0x22/0xd7
> > > [ 154.618310] Code: cc cc cc cc cc cc cc cc 0f 1f 44 00 00 66 90 55 48 89 e5 48 81 ec 30 00 00 00 53 41 55 41 56 48 89 fb 4c 8b 6b 00 4c 8b 73 08 <41> 8b be 0c 04 00 00 48 83 ff 06 0f 85 9b 00 00 00 41 8b be c0 09
> > > [ 154.637052] RSP: 0018:ffffabac60aebbc0 EFLAGS: 00010086
> > > [ 154.642278] RAX: ffffffffc03fba5c RBX: ffffabac60aebc28 RCX: 000000000000001f
> > > [ 154.649411] RDX: ffff95a90b4e4180 RSI: ffffabac4e639048 RDI: ffffabac60aebc28
> > > [ 154.656544] RBP: ffffabac60aebc08 R08: 00000023fce7674a R09: ffff95a91d85af38
> > > [ 154.663674] R10: ffff95a91d85a0c0 R11: 000000003357e518 R12: 0000000000000000
> > > [ 154.670807] R13: ffff95a90b4e4180 R14: 0000000000000000 R15: 0000000000000001
> > > [ 154.677939] FS: 00007ffa6d600640(0000) GS:ffff95c01bf00000(0000) knlGS:0000000000000000
> > > [ 154.686026] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > > [ 154.691769] CR2: 000000000000040c CR3: 000000014b9f2005 CR4: 00000000007706f0
> > > [ 154.698903] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > > [ 154.706035] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> > > [ 154.713168] PKRU: 55555554
> > > [ 154.715879] Call Trace:
> > > [ 154.718332] <TASK>
> > > [ 154.720439] ? __die+0x20/0x70
> > > [ 154.723498] ? page_fault_oops+0x75/0x170
> > > [ 154.727508] ? sysvec_irq_work+0xb/0x90
> > > [ 154.731348] ? exc_page_fault+0x64/0x140
> > > [ 154.735275] ? asm_exc_page_fault+0x22/0x30
> > > [ 154.739461] ? 0xffffffffc03fba5c
> > > [ 154.742780] ? bpf_prog_ec8173ca2868eb50_handle__sched_pi_setprio+0x22/0xd7
> >
> > hi,
> > reproduced.. AFAICS looks like the bpf program somehow lost the booster != NULL
> > check and just load the policy field without it and crash when booster is rubbish
> >
> > int handle__sched_pi_setprio(u64 * ctx):
> > ; int handle__sched_pi_setprio(u64 *ctx)
> > 0: (bf) r6 = r1
> > ; struct task_struct *boosted = (void *) ctx[0];
> > 1: (79) r7 = *(u64 *)(r6 +0)
> > ; struct task_struct *booster = (void *) ctx[1];
> > 2: (79) r8 = *(u64 *)(r6 +8)
> > ; if (booster->policy != SCHED_DEADLINE)
> >
> > curious why the check disappeared, because object file has it, so I guess verifier
> > took it out for some reason, will check
>
> Juri,
>
> Thanks for flagging!
>
> Jiri,
>
> the verifier removes the check because it assumes that pointers
> passed by the kernel into tracepoint are valid and trusted.
> In this case:
> trace_sched_pi_setprio(p, pi_task);
>
> pi_task can be NULL.
>
> We cannot make all tracepoint pointers to be PTR_TRUSTED | PTR_MAYBE_NULL
> by default, since it will break a bunch of progs.
> Instead we can annotate this tracepoint arg as __nullable and
> teach the verifier to recognize such special arguments of tracepoints.
ok, so you mean to be able to mark it in event header like:
TRACE_EVENT(sched_pi_setprio,
TP_PROTO(struct task_struct *tsk, struct task_struct *pi_task __nullable),
I guess we could make pahole to emit DECL_TAG for that argument,
but I'm not sure how to propagate that __nullable info to pahole
while wondering about that, I tried the direct fix below ;-)
jirka
---
diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 95426d5b634e..1a20bbdead64 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -6377,6 +6377,25 @@ int btf_ctx_arg_offset(const struct btf *btf, const struct btf_type *func_proto,
return off;
}
+static bool is_tracing_prog_raw_tp(const struct bpf_prog *prog, const char *name)
+{
+ struct btf *btf = prog->aux->attach_btf;
+ const struct btf_type *t;
+ const char *tname;
+
+ if (prog->expected_attach_type != BPF_TRACE_RAW_TP)
+ return false;
+
+ t = btf_type_by_id(btf, prog->aux->attach_btf_id);
+ if (!t)
+ return false;
+
+ tname = btf_name_by_offset(btf, t->name_off);
+ if (!tname)
+ return false;
+ return !strcmp(tname, name);
+}
+
bool btf_ctx_access(int off, int size, enum bpf_access_type type,
const struct bpf_prog *prog,
struct bpf_insn_access_aux *info)
@@ -6544,6 +6563,10 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
}
}
+ /* Second argument of sched_pi_setprio tracepoint can be null */
+ if (is_tracing_prog_raw_tp(prog, "btf_trace_sched_pi_setprio") && arg == 1)
+ info->reg_type |= PTR_MAYBE_NULL;
+
info->btf = btf;
info->btf_id = t->type;
t = btf_type_by_id(btf, t->type);
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