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Subject[PATCH v2 bpf-next] bpf: sk_storage: Fix out of bounds memory access
Date
bpf_sk_storage maps use multiple spin locks to reduce contention.
The number of locks to use is determined by the number of possible CPUs.
With only 1 possible CPU, bucket_log == 0, and 2^0 = 1 locks are used.

When updating elements, the correct lock is determined with hash_ptr().
Calling hash_ptr() with 0 bits is undefined behavior, as it does:

x >> (64 - bits)

Using the value results in an out of bounds memory access.
In my case, this manifested itself as a page fault when raw_spin_lock_bh()
is called later, when running the self tests:

./tools/testing/selftests/bpf/test_verifier 773 775

[ 16.366342] BUG: unable to handle page fault for address: ffff8fe7a66f93f8
[ 16.367139] #PF: supervisor write access in kernel mode
[ 16.367751] #PF: error_code(0x0002) - not-present page
[ 16.368323] PGD 35a01067 P4D 35a01067 PUD 0
[ 16.368796] Oops: 0002 [#1] SMP PTI
[ 16.369175] CPU: 0 PID: 189 Comm: test_verifier Not tainted 5.2.0-rc2+ #10
[ 16.369960] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014
[ 16.371021] RIP: 0010:_raw_spin_lock_bh (/home/afabre/linux/./include/trace/events/initcall.h:48)
[ 16.371571] Code: 02 00 00 31 c0 ba ff 00 00 00 3e 0f b1 17 75 01 c3 e9 82 12 5f ff 66 90 65 81 05 ad 14 6f 41 00 02 00 00 31 c0 ba 01 00 00 00 <3e> 0f b1 17 75 01 c3 89 c6 e9 f0 02 5f ff b8 00 02 00 00 3e 0f c1
All code
========
0: 02 00 add (%rax),%al
2: 00 31 add %dh,(%rcx)
4: c0 ba ff 00 00 00 3e sarb $0x3e,0xff(%rdx)
b: 0f b1 17 cmpxchg %edx,(%rdi)
e: 75 01 jne 0x11
10: c3 retq
11: e9 82 12 5f ff jmpq 0xffffffffff5f1298
16: 66 90 xchg %ax,%ax
18: 65 81 05 ad 14 6f 41 addl $0x200,%gs:0x416f14ad(%rip) # 0x416f14d0
1f: 00 02 00 00
23: 31 c0 xor %eax,%eax
25: ba 01 00 00 00 mov $0x1,%edx
2a: 3e 0f b1 17 cmpxchg %edx,%ds:*(%rdi) <-- trapping instruction
2e: 75 01 jne 0x31
30: c3 retq
31: 89 c6 mov %eax,%esi
33: e9 f0 02 5f ff jmpq 0xffffffffff5f0328
38: b8 00 02 00 00 mov $0x200,%eax
3d: 3e ds
3e: 0f .byte 0xf
3f: c1 .byte 0xc1

Code starting with the faulting instruction
===========================================
0: 3e 0f b1 17 cmpxchg %edx,%ds:(%rdi)
4: 75 01 jne 0x7
6: c3 retq
7: 89 c6 mov %eax,%esi
9: e9 f0 02 5f ff jmpq 0xffffffffff5f02fe
e: b8 00 02 00 00 mov $0x200,%eax
13: 3e ds
14: 0f .byte 0xf
15: c1 .byte 0xc1
[ 16.373398] RSP: 0018:ffffa759809d3be0 EFLAGS: 00010246
[ 16.373954] RAX: 0000000000000000 RBX: ffff8fe7a66f93f0 RCX: 0000000000000040
[ 16.374645] RDX: 0000000000000001 RSI: ffff8fdaf9f0d180 RDI: ffff8fe7a66f93f8
[ 16.375338] RBP: ffff8fdaf9f0d180 R08: ffff8fdafba2c320 R09: ffff8fdaf9f0d0c0
[ 16.376028] R10: 0000000000000000 R11: 0000000000000000 R12: ffff8fdafa346700
[ 16.376719] R13: ffff8fe7a66f93f8 R14: ffff8fdaf9f0d0c0 R15: 0000000000000001
[ 16.377413] FS: 00007fda724c0740(0000) GS:ffff8fdafba00000(0000) knlGS:0000000000000000
[ 16.378204] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 16.378763] CR2: ffff8fe7a66f93f8 CR3: 0000000139d1c006 CR4: 0000000000360ef0
[ 16.379453] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 16.380144] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[ 16.380864] Call Trace:
[ 16.381112] selem_link_map (/home/afabre/linux/./include/linux/compiler.h:221 /home/afabre/linux/net/core/bpf_sk_storage.c:243)
[ 16.381476] sk_storage_update (/home/afabre/linux/net/core/bpf_sk_storage.c:355 /home/afabre/linux/net/core/bpf_sk_storage.c:414)
[ 16.381888] bpf_sk_storage_get (/home/afabre/linux/net/core/bpf_sk_storage.c:760 /home/afabre/linux/net/core/bpf_sk_storage.c:741)
[ 16.382285] ___bpf_prog_run (/home/afabre/linux/kernel/bpf/core.c:1447)
[ 16.382679] ? __bpf_prog_run32 (/home/afabre/linux/kernel/bpf/core.c:1603)
[ 16.383074] ? alloc_file_pseudo (/home/afabre/linux/fs/file_table.c:232)
[ 16.383486] ? kvm_clock_get_cycles (/home/afabre/linux/arch/x86/kernel/kvmclock.c:98)
[ 16.383906] ? ktime_get (/home/afabre/linux/kernel/time/timekeeping.c:265 /home/afabre/linux/kernel/time/timekeeping.c:369 /home/afabre/linux/kernel/time/timekeeping.c:754)
[ 16.384243] ? bpf_test_run (/home/afabre/linux/net/bpf/test_run.c:47)
[ 16.384613] ? bpf_prog_test_run_skb (/home/afabre/linux/net/bpf/test_run.c:313)
[ 16.385065] ? security_capable (/home/afabre/linux/security/security.c:696 (discriminator 19))
[ 16.385460] ? __do_sys_bpf (/home/afabre/linux/kernel/bpf/syscall.c:2072 /home/afabre/linux/kernel/bpf/syscall.c:2848)
[ 16.385854] ? __handle_mm_fault (/home/afabre/linux/mm/memory.c:3507 /home/afabre/linux/mm/memory.c:3532 /home/afabre/linux/mm/memory.c:3666 /home/afabre/linux/mm/memory.c:3897 /home/afabre/linux/mm/memory.c:4021)
[ 16.386273] ? __dentry_kill (/home/afabre/linux/fs/dcache.c:595)
[ 16.386652] ? do_syscall_64 (/home/afabre/linux/arch/x86/entry/common.c:301)
[ 16.387031] ? entry_SYSCALL_64_after_hwframe (/home/afabre/linux/./include/trace/events/initcall.h:10 /home/afabre/linux/./include/trace/events/initcall.h:10)
[ 16.387541] Modules linked in:
[ 16.387846] CR2: ffff8fe7a66f93f8
[ 16.388175] ---[ end trace 891cf27b5b9c9cc6 ]---
[ 16.388628] RIP: 0010:_raw_spin_lock_bh (/home/afabre/linux/./include/trace/events/initcall.h:48)
[ 16.389089] Code: 02 00 00 31 c0 ba ff 00 00 00 3e 0f b1 17 75 01 c3 e9 82 12 5f ff 66 90 65 81 05 ad 14 6f 41 00 02 00 00 31 c0 ba 01 00 00 00 <3e> 0f b1 17 75 01 c3 89 c6 e9 f0 02 5f ff b8 00 02 00 00 3e 0f c1
All code
========
0: 02 00 add (%rax),%al
2: 00 31 add %dh,(%rcx)
4: c0 ba ff 00 00 00 3e sarb $0x3e,0xff(%rdx)
b: 0f b1 17 cmpxchg %edx,(%rdi)
e: 75 01 jne 0x11
10: c3 retq
11: e9 82 12 5f ff jmpq 0xffffffffff5f1298
16: 66 90 xchg %ax,%ax
18: 65 81 05 ad 14 6f 41 addl $0x200,%gs:0x416f14ad(%rip) # 0x416f14d0
1f: 00 02 00 00
23: 31 c0 xor %eax,%eax
25: ba 01 00 00 00 mov $0x1,%edx
2a: 3e 0f b1 17 cmpxchg %edx,%ds:*(%rdi) <-- trapping instruction
2e: 75 01 jne 0x31
30: c3 retq
31: 89 c6 mov %eax,%esi
33: e9 f0 02 5f ff jmpq 0xffffffffff5f0328
38: b8 00 02 00 00 mov $0x200,%eax
3d: 3e ds
3e: 0f .byte 0xf
3f: c1 .byte 0xc1

Code starting with the faulting instruction
===========================================
0: 3e 0f b1 17 cmpxchg %edx,%ds:(%rdi)
4: 75 01 jne 0x7
6: c3 retq
7: 89 c6 mov %eax,%esi
9: e9 f0 02 5f ff jmpq 0xffffffffff5f02fe
e: b8 00 02 00 00 mov $0x200,%eax
13: 3e ds
14: 0f .byte 0xf
15: c1 .byte 0xc1
[ 16.390899] RSP: 0018:ffffa759809d3be0 EFLAGS: 00010246
[ 16.391410] RAX: 0000000000000000 RBX: ffff8fe7a66f93f0 RCX: 0000000000000040
[ 16.392102] RDX: 0000000000000001 RSI: ffff8fdaf9f0d180 RDI: ffff8fe7a66f93f8
[ 16.392795] RBP: ffff8fdaf9f0d180 R08: ffff8fdafba2c320 R09: ffff8fdaf9f0d0c0
[ 16.393481] R10: 0000000000000000 R11: 0000000000000000 R12: ffff8fdafa346700
[ 16.394169] R13: ffff8fe7a66f93f8 R14: ffff8fdaf9f0d0c0 R15: 0000000000000001
[ 16.394870] FS: 00007fda724c0740(0000) GS:ffff8fdafba00000(0000) knlGS:0000000000000000
[ 16.395641] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 16.396193] CR2: ffff8fe7a66f93f8 CR3: 0000000139d1c006 CR4: 0000000000360ef0
[ 16.396876] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 16.397557] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[ 16.398246] Kernel panic - not syncing: Fatal exception in interrupt
[ 16.399067] Kernel Offset: 0x3ce00000 from 0xffffffff81000000 (relocation range: 0xffffffff80000000-0xffffffffbfffffff)
[ 16.400098] ---[ end Kernel panic - not syncing: Fatal exception in interrupt ]---

Force the minimum number of locks to two.

Signed-off-by: Arthur Fabre <afabre@cloudflare.com>
Fixes: 6ac99e8f23d4 ("bpf: Introduce bpf sk local storage")
---
net/core/bpf_sk_storage.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/core/bpf_sk_storage.c b/net/core/bpf_sk_storage.c
index f40e3d35fd9c..94c7f77ecb6b 100644
--- a/net/core/bpf_sk_storage.c
+++ b/net/core/bpf_sk_storage.c
@@ -634,7 +634,8 @@ static struct bpf_map *bpf_sk_storage_map_alloc(union bpf_attr *attr)
return ERR_PTR(-ENOMEM);
bpf_map_init_from_attr(&smap->map, attr);

- smap->bucket_log = ilog2(roundup_pow_of_two(num_possible_cpus()));
+ /* Use at least 2 buckets, select_bucket() is undefined behavior with 1 bucket */
+ smap->bucket_log = max_t(u32, 1, ilog2(roundup_pow_of_two(num_possible_cpus())));
nbuckets = 1U << smap->bucket_log;
cost = sizeof(*smap->buckets) * nbuckets + sizeof(*smap);

--
2.20.1
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