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linux-next/kernel/bpf
Daniel Borkmann f54c7898ed bpf: Fix precision tracking for unbounded scalars
Anatoly has been fuzzing with kBdysch harness and reported a hang in one
of the outcomes. Upon closer analysis, it turns out that precise scalar
value tracking is missing a few precision markings for unknown scalars:

  0: R1=ctx(id=0,off=0,imm=0) R10=fp0
  0: (b7) r0 = 0
  1: R0_w=invP0 R1=ctx(id=0,off=0,imm=0) R10=fp0
  1: (35) if r0 >= 0xf72e goto pc+0
  --> only follow fallthrough
  2: R0_w=invP0 R1=ctx(id=0,off=0,imm=0) R10=fp0
  2: (35) if r0 >= 0x80fe0000 goto pc+0
  --> only follow fallthrough
  3: R0_w=invP0 R1=ctx(id=0,off=0,imm=0) R10=fp0
  3: (14) w0 -= -536870912
  4: R0_w=invP536870912 R1=ctx(id=0,off=0,imm=0) R10=fp0
  4: (0f) r1 += r0
  5: R0_w=invP536870912 R1_w=inv(id=0) R10=fp0
  5: (55) if r1 != 0x104c1500 goto pc+0
  --> push other branch for later analysis
  R0_w=invP536870912 R1_w=inv273421568 R10=fp0
  6: R0_w=invP536870912 R1_w=inv273421568 R10=fp0
  6: (b7) r0 = 0
  7: R0=invP0 R1=inv273421568 R10=fp0
  7: (76) if w1 s>= 0xffffff00 goto pc+3
  --> only follow goto
  11: R0=invP0 R1=inv273421568 R10=fp0
  11: (95) exit
  6: R0_w=invP536870912 R1_w=inv(id=0) R10=fp0
  6: (b7) r0 = 0
  propagating r0
  7: safe
  processed 11 insns [...]

In the analysis of the second path coming after the successful exit above,
the path is being pruned at line 7. Pruning analysis found that both r0 are
precise P0 and both R1 are non-precise scalars and given prior path with
R1 as non-precise scalar succeeded, this one is therefore safe as well.

However, problem is that given condition at insn 7 in the first run, we only
followed goto and didn't push the other branch for later analysis, we've
never walked the few insns in there and therefore dead-code sanitation
rewrites it as goto pc-1, causing the hang depending on the skb address
hitting these conditions. The issue is that R1 should have been marked as
precise as well such that pruning enforces range check and conluded that new
R1 is not in range of old R1. In insn 4, we mark R1 (skb) as unknown scalar
via __mark_reg_unbounded() but not mark_reg_unbounded() and therefore
regs->precise remains as false.

Back in b5dc0163d8 ("bpf: precise scalar_value tracking"), this was not
the case since marking out of __mark_reg_unbounded() had this covered as well.
Once in both are set as precise in 4 as they should have been, we conclude
that given R1 was in prior fall-through path 0x104c1500 and now is completely
unknown, the check at insn 7 concludes that we need to continue walking.
Analysis after the fix:

  0: R1=ctx(id=0,off=0,imm=0) R10=fp0
  0: (b7) r0 = 0
  1: R0_w=invP0 R1=ctx(id=0,off=0,imm=0) R10=fp0
  1: (35) if r0 >= 0xf72e goto pc+0
  2: R0_w=invP0 R1=ctx(id=0,off=0,imm=0) R10=fp0
  2: (35) if r0 >= 0x80fe0000 goto pc+0
  3: R0_w=invP0 R1=ctx(id=0,off=0,imm=0) R10=fp0
  3: (14) w0 -= -536870912
  4: R0_w=invP536870912 R1=ctx(id=0,off=0,imm=0) R10=fp0
  4: (0f) r1 += r0
  5: R0_w=invP536870912 R1_w=invP(id=0) R10=fp0
  5: (55) if r1 != 0x104c1500 goto pc+0
  R0_w=invP536870912 R1_w=invP273421568 R10=fp0
  6: R0_w=invP536870912 R1_w=invP273421568 R10=fp0
  6: (b7) r0 = 0
  7: R0=invP0 R1=invP273421568 R10=fp0
  7: (76) if w1 s>= 0xffffff00 goto pc+3
  11: R0=invP0 R1=invP273421568 R10=fp0
  11: (95) exit
  6: R0_w=invP536870912 R1_w=invP(id=0) R10=fp0
  6: (b7) r0 = 0
  7: R0_w=invP0 R1_w=invP(id=0) R10=fp0
  7: (76) if w1 s>= 0xffffff00 goto pc+3
  R0_w=invP0 R1_w=invP(id=0) R10=fp0
  8: R0_w=invP0 R1_w=invP(id=0) R10=fp0
  8: (a5) if r0 < 0x2007002a goto pc+0
  9: R0_w=invP0 R1_w=invP(id=0) R10=fp0
  9: (57) r0 &= -16316416
  10: R0_w=invP0 R1_w=invP(id=0) R10=fp0
  10: (a6) if w0 < 0x1201 goto pc+0
  11: R0_w=invP0 R1_w=invP(id=0) R10=fp0
  11: (95) exit
  11: R0=invP0 R1=invP(id=0) R10=fp0
  11: (95) exit
  processed 16 insns [...]

Fixes: 6754172c20 ("bpf: fix precision tracking in presence of bpf2bpf calls")
Reported-by: Anatoly Trosinenko <anatoly.trosinenko@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20191222223740.25297-1-daniel@iogearbox.net
2019-12-22 17:21:10 -08:00
..
arraymap.c bpf: Simplify __bpf_arch_text_poke poke type handling 2019-11-24 17:12:11 -08:00
bpf_lru_list.c treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 206 2019-05-30 11:29:53 -07:00
bpf_lru_list.h treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 206 2019-05-30 11:29:53 -07:00
btf.c bpf: Fix build in minimal configurations, again 2019-12-11 13:57:26 +01:00
cgroup.c treewide: Use sizeof_field() macro 2019-12-09 10:36:44 -08:00
core.c bpf: Fix cgroup local storage prog tracking 2019-12-17 08:58:02 -08:00
cpumap.c devmap/cpumap: Use flush list instead of bitmap 2019-06-29 01:31:08 +02:00
devmap.c xdp: Fix cleanup on map free for devmap_hash map type 2019-11-24 16:58:46 -08:00
disasm.c treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 295 2019-06-05 17:36:38 +02:00
disasm.h treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 295 2019-06-05 17:36:38 +02:00
hashtab.c Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net 2019-06-17 20:20:36 -07:00
helpers.c cgroup: use cgrp->kn->id as the cgroup ID 2019-11-12 08:18:04 -08:00
inode.c bpf: Convert bpf_prog refcnt to atomic64_t 2019-11-18 11:41:59 +01:00
local_storage.c Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net 2019-12-22 09:54:33 -08:00
lpm_trie.c Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net 2019-06-22 08:59:24 -04:00
Makefile bpf: Introduce BPF trampoline 2019-11-15 23:41:51 +01:00
map_in_map.c bpf: Move owner type, jited info into array auxiliary data 2019-11-24 17:04:11 -08:00
map_in_map.h treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 206 2019-05-30 11:29:53 -07:00
offload.c bpf, offload: Unlock on error in bpf_offload_dev_create() 2019-11-07 00:20:27 +01:00
percpu_freelist.c treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 206 2019-05-30 11:29:53 -07:00
percpu_freelist.h treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 206 2019-05-30 11:29:53 -07:00
queue_stack_maps.c bpf: move memory size checks to bpf_map_charge_init() 2019-05-31 16:52:56 -07:00
reuseport_array.c bpf: move memory size checks to bpf_map_charge_init() 2019-05-31 16:52:56 -07:00
stackmap.c Merge branch 'irq-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2019-12-03 09:29:50 -08:00
syscall.c bpf: Add poke dependency tracking for prog array maps 2019-11-24 17:04:11 -08:00
sysfs_btf.c btf: fix return value check in btf_vmlinux_init() 2019-08-15 22:18:17 -07:00
tnum.c treewide: Add SPDX license identifier for missed files 2019-05-21 10:50:45 +02:00
trampoline.c bpf: Make BPF trampoline use register_ftrace_direct() API 2019-12-11 15:18:08 -08:00
verifier.c bpf: Fix precision tracking for unbounded scalars 2019-12-22 17:21:10 -08:00
xskmap.c bpf: Switch bpf_map ref counter to atomic64_t so bpf_map_inc() never fails 2019-11-18 11:41:59 +01:00