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Message-ID: <ZvWN0_FNCCJ7Y6d7@archlinux> Date: Thu, 26 Sep 2024 18:37:39 +0200 From: Jan Hendrik Farr <kernel@...rr.cc> To: Thorsten Blum <thorsten.blum@...lux.com> Cc: kent.overstreet@...ux.dev, regressions@...ts.linux.dev, linux-bcachefs@...r.kernel.org, linux-hardening@...r.kernel.org, linux-kernel@...r.kernel.org Subject: Re: [REGRESSION][BISECTED] erroneous buffer overflow detected in bch2_xattr_validate On 26 18:09:57, Thorsten Blum wrote: > On 26. Sep 2024, at 17:28, Thorsten Blum <thorsten.blum@...lux.com> wrote: > > On 26. Sep 2024, at 17:14, Jan Hendrik Farr <kernel@...rr.cc> wrote: > >> > >> Hi Kent, > >> > >> found a strange regression in the patch set for 6.12. > >> > >> First bad commit is: 86e92eeeb23741a072fe7532db663250ff2e726a > >> bcachefs: Annotate struct bch_xattr with __counted_by() > >> > >> When compiling with clang 18.1.8 (also with latest llvm main branch) and > >> CONFIG_FORTIFY_SOURCE=y my rootfs does not mount because there is an erroneous > >> detection of a buffer overflow. > >> > >> The __counted_by attribute is supposed to be supported starting with gcc 15, > >> not sure if it is implemented yet so I haven't tested with gcc trunk yet. > >> > >> Here's the relevant section of dmesg: > >> > >> [ 6.248736] bcachefs (nvme1n1p2): starting version 1.12: rebalance_work_acct_fix > >> [ 6.248744] bcachefs (nvme1n1p2): recovering from clean shutdown, journal seq 1305969 > >> [ 6.252374] ------------[ cut here ]------------ > >> [ 6.252375] memchr: detected buffer overflow: 12 byte read of buffer size 0 > >> [ 6.252379] WARNING: CPU: 18 PID: 511 at lib/string_helpers.c:1033 __fortify_report+0x45/0x50 > >> [ 6.252383] Modules linked in: bcachefs lz4hc_compress lz4_compress hid_generic usbhid btrfs crct10dif_pclmul libcrc32c crc32_pclmul crc32c_generic polyval_clmulni crc32c_intel polyval_generic raid6_pq ghash_clmulni_intel xor sha512_ssse3 sha256_ssse3 sha1_ssse3 aesni_intel gf128mul nvme crypto_simd ccp xhci_pci cryptd sp5100_tco xhci_pci_renesas nvme_core nvme_auth video wmi ip6_tables ip_tables x_tables i2c_dev > >> [ 6.252404] CPU: 18 UID: 0 PID: 511 Comm: mount Not tainted 6.11.0-10065-g6fa6588e5964 #98 d8e0beb515d91b387aa60970de7203f35ddd182c > >> [ 6.252406] Hardware name: Micro-Star International Co., Ltd. MS-7D78/PRO B650-P WIFI (MS-7D78), BIOS 1.C0 02/06/2024 > >> [ 6.252407] RIP: 0010:__fortify_report+0x45/0x50 > >> [ 6.252409] Code: 48 8b 34 c5 30 92 21 87 40 f6 c7 01 48 c7 c0 75 1b 0a 87 48 c7 c1 e1 93 07 87 48 0f 44 c8 48 c7 c7 ef 03 10 87 e8 0b c2 9b ff <0f> 0b e9 cf 5d 9e 00 cc cc cc cc 90 90 90 90 90 90 90 90 90 90 90 > >> [ 6.252410] RSP: 0018:ffffbb3d03aff350 EFLAGS: 00010246 > >> [ 6.252412] RAX: 4ce590fb7c372800 RBX: ffff98d559a400e8 RCX: 0000000000000027 > >> [ 6.252413] RDX: 0000000000000002 RSI: 00000000ffffdfff RDI: ffff98e43db21a08 > >> [ 6.252414] RBP: ffff98d559a400d0 R08: 0000000000001fff R09: ffff98e47ddcd000 > >> [ 6.252415] R10: 0000000000005ffd R11: 0000000000000004 R12: ffff98d559a40000 > >> [ 6.252416] R13: ffff98d54abf1320 R14: ffffbb3d03aff430 R15: 0000000000000000 > >> [ 6.252417] FS: 00007efc82117800(0000) GS:ffff98e43db00000(0000) knlGS:0000000000000000 > >> [ 6.252418] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 > >> [ 6.252419] CR2: 000055d96658ea80 CR3: 000000010a12c000 CR4: 0000000000f50ef0 > >> [ 6.252420] PKRU: 55555554 > >> [ 6.252421] Call Trace: > >> [ 6.252423] <TASK> > >> [ 6.252425] ? __warn+0xd5/0x1d0 > >> [ 6.252427] ? __fortify_report+0x45/0x50 > >> [ 6.252429] ? report_bug+0x144/0x1f0 > >> [ 6.252431] ? __fortify_report+0x45/0x50 > >> [ 6.252433] ? handle_bug+0x6a/0x90 > >> [ 6.252435] ? exc_invalid_op+0x1a/0x50 > >> [ 6.252436] ? asm_exc_invalid_op+0x1a/0x20 > >> [ 6.252440] ? __fortify_report+0x45/0x50 > >> [ 6.252441] __fortify_panic+0x9/0x10 > >> [ 6.252443] bch2_xattr_validate+0x13b/0x140 [bcachefs 8361179bbfcc59e669df38aec976f02d7211a659] > >> [ 6.252463] bch2_btree_node_read_done+0x125a/0x17a0 [bcachefs 8361179bbfcc59e669df38aec976f02d7211a659] > >> [ 6.252482] btree_node_read_work+0x202/0x4a0 [bcachefs 8361179bbfcc59e669df38aec976f02d7211a659] > >> [ 6.252499] bch2_btree_node_read+0xa8d/0xb20 [bcachefs 8361179bbfcc59e669df38aec976f02d7211a659] > >> [ 6.252514] ? srso_alias_return_thunk+0x5/0xfbef5 > >> [ 6.252515] ? pcpu_alloc_noprof+0x741/0xb50 > >> [ 6.252517] ? srso_alias_return_thunk+0x5/0xfbef5 > >> [ 6.252519] ? time_stats_update_one+0x75/0x1f0 [bcachefs 8361179bbfcc59e669df38aec976f02d7211a659] > >> > >> ... > >> > >> > >> The memchr in question is at: > >> https://github.com/torvalds/linux/blob/11a299a7933e03c83818b431e6a1c53ad387423d/fs/bcachefs/xattr.c#L99 > >> > >> There is not actually a buffer overflow here, I checked with gdb that > >> xattr.v->x_name does actually contain a string of the correct length and > >> xattr.v->x_name_len contains the correct length and should be used to determine > >> the length when memchr uses __struct_size for bounds-checking due to the > >> __counted_by annotation. > >> > >> I'm at the point where I think this is probably a bug in clang. I have a patch > >> that does fix (more like bandaid) the problem and adds some print statements: > >> > >> -- > >> diff --git a/fs/bcachefs/xattr.c b/fs/bcachefs/xattr.c > >> index 56c8d3fe55a4..8d7e749b7dda 100644 > >> --- a/fs/bcachefs/xattr.c > >> +++ b/fs/bcachefs/xattr.c > >> @@ -74,6 +74,7 @@ int bch2_xattr_validate(struct bch_fs *c, struct bkey_s_c k, > >> enum bch_validate_flags flags) > >> { > >> struct bkey_s_c_xattr xattr = bkey_s_c_to_xattr(k); > >> + const struct bch_xattr *v = (void *)k.v; > >> unsigned val_u64s = xattr_val_u64s(xattr.v->x_name_len, > >> le16_to_cpu(xattr.v->x_val_len)); > >> int ret = 0; > >> @@ -94,9 +95,12 @@ int bch2_xattr_validate(struct bch_fs *c, struct bkey_s_c k, > >> > >> bkey_fsck_err_on(!bch2_xattr_type_to_handler(xattr.v->x_type), > >> c, xattr_invalid_type, > >> - "invalid type (%u)", xattr.v->x_type); > >> + "invalid type (%u)", v->x_type); > >> > >> - bkey_fsck_err_on(memchr(xattr.v->x_name, '\0', xattr.v->x_name_len), > >> + pr_info("x_name_len: %d", v->x_name_len); > >> + pr_info("__struct_size(x_name): %ld", __struct_size(v->x_name)); > >> + pr_info("__struct_size(x_name): %ld", __struct_size(xattr.v->x_name)); > >> + bkey_fsck_err_on(memchr(v->x_name, '\0', v->x_name_len), > >> c, xattr_name_invalid_chars, > >> "xattr name has invalid characters"); > >> fsck_err: > >> -- > >> > >> > >> Making memchr access via a pointer created with > >> const struct bch_xattr *v = (void *)k.v fixes it. From the print statements I > >> can see that __struct_size(xattr.v->x_name) incorrectly returns 0, while > >> __struct_size(v->x_name) correctly returns 10 in this case (the value of > >> x_name_len). > >> > >> The generated assembly illustrates what is going wrong. Below is an excerpt > >> of the assembly clang generated for the bch2_xattr_validate function: > >> > >> mov r13d, ecx > >> mov r15, rdi > >> mov r14, rsi > >> mov rdi, offset .L.str.3 > >> mov rsi, offset .L__func__.bch2_xattr_validate > >> mov rbx, rdx > >> mov edx, eax > >> call _printk > >> movzx edx, byte ptr [rbx + 1] > >> mov rdi, offset .L.str.4 > >> mov rsi, offset .L__func__.bch2_xattr_validate > >> call _printk > >> movzx edx, bh > >> mov rdi, offset .L.str.4 > >> mov rsi, offset .L__func__.bch2_xattr_validate > >> call _printk > >> lea rdi, [rbx + 4] > >> mov r12, rbx > >> movzx edx, byte ptr [rbx + 1] > >> xor ebx, ebx > >> xor esi, esi > >> call memchr > >> > >> At the start of this rdx contains k.v (and is moved into rbx). The three calls > >> to printk are the ones you can see in my patch. You can see that for the > >> print that uses __struct_size(v->x_name) the compiler correctly uses > >> movzx edx, byte ptr [rbx + 1] > >> to load x_name_len into edx. > >> > >> For the printk call that uses __struct_size(xattr.v->x_name) however the > >> compiler uses > >> movzx edx, bh > >> So it will print the high 8 bits of the lower 16 bits (second least > >> significant byte) of the memory address of xattr.v->x_type. This is obviously > >> completely wrong. > >> > >> It is then doing the correct call of memchr because this is using my patch. > >> Without my patch it would be doing the same thing for the call to memchr where > >> it uses the second least significant byte of the memory address of x_type as the > >> length used for the bounds-check. > >> > >> > >> > >> The LLVM IR also shows the same problem: > >> > >> define internal zeroext i1 @xattr_cmp_key(ptr nocapture readnone %0, ptr %1, ptr nocapture noundef readonly %2) #0 align 16 { > >> [...] > >> %51 = ptrtoint ptr %2 to i64 > >> %52 = lshr i64 %51, 8 > >> %53 = and i64 %52, 255 > >> > >> This is the IR for the incorrect behavior. It simply converts the pointer to an > >> int, shifts right by 8 bits, then and with 0xFF. If it did a load (to i64) > >> instead of ptrtoint this would actually work, as the second least significant > >> bit of an i64 loaded from that memory address does contain the value of > >> x_name_len. It's as if clang forgot to dereference a pointer here. > >> > >> Correct IR does this (for the other printk invocation): > >> > >> define internal zeroext i1 @xattr_cmp_key(ptr nocapture readnone %0, ptr %1, ptr nocapture noundef readonly %2) #0 align 16 { > >> [...] > >> %4 = getelementptr inbounds %struct.bch_xattr, ptr %1, i64 0, i32 1 > >> %5 = load i8, ptr %4, align 8 > >> [...] > >> %48 = load i8, ptr %5, align 4 > >> %49 = zext i8 %48 to i64 > >> > >> Best Regards > >> Jan > > > > I suspect it's the same Clang __bdos() "bug" as in [1] and [2]. > > > > [1] https://lore.kernel.org/linux-kernel/3D0816D1-0807-4D37-8D5F-3C55CA910FAA@linux.dev/ > > [2] https://lore.kernel.org/all/20240913164630.GA4091534@thelio-3990X/ > > Could you try this and see if it resolves the problem? > > diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h > index 1a957ea2f4fe..b09759f31789 100644 > --- a/include/linux/compiler_types.h > +++ b/include/linux/compiler_types.h > @@ -413,7 +413,7 @@ struct ftrace_likely_data { > * When the size of an allocated object is needed, use the best available > * mechanism to find it. (For cases where sizeof() cannot be used.) > */ > -#if __has_builtin(__builtin_dynamic_object_size) > +#if __has_builtin(__builtin_dynamic_object_size) && !defined(__clang__) > #define __struct_size(p) __builtin_dynamic_object_size(p, 0) > #define __member_size(p) __builtin_dynamic_object_size(p, 1) > #else > Weirdly enough it does not. If I print the result of __struct_size before the call to memchr I get 0xFFFF... though, so it should work. But in memchr it still get's 0. I'll fire up the debugger... > Thanks, > Thorsten
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