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Thu, 26 Sep 2024 13:45:54 -0400 (EDT) Date: Thu, 26 Sep 2024 19:45:53 +0200 From: Jan Hendrik Farr To: Thorsten Blum Cc: kent.overstreet@linux.dev, regressions@lists.linux.dev, linux-bcachefs@vger.kernel.org, linux-hardening@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [REGRESSION][BISECTED] erroneous buffer overflow detected in bch2_xattr_validate Message-ID: References: <3E304FB2-799D-478F-889A-CDFC1A52DCD8@toblux.com> Precedence: bulk X-Mailing-List: linux-bcachefs@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On 26 19:01:20, Jan Hendrik Farr wrote: > On 26 18:09:57, Thorsten Blum wrote: > > On 26. Sep 2024, at 17:28, Thorsten Blum wrote: > > > On 26. Sep 2024, at 17:14, Jan Hendrik Farr 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] > > >> [ 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 > > > > Alright after looking at it in the debugger the code it generates now is > just wild. > > I added one more printk before the call to memchr like so: > > diff --git a/fs/bcachefs/xattr.c b/fs/bcachefs/xattr.c > index 56c8d3fe55a4..3c7c479ea3a8 100644 > --- a/fs/bcachefs/xattr.c > +++ b/fs/bcachefs/xattr.c > @@ -96,6 +96,7 @@ int bch2_xattr_validate(struct bch_fs *c, struct bkey_s_c k, > c, xattr_invalid_type, > "invalid type (%u)", xattr.v->x_type); > > + pr_info("__struct_size(x_name): %lu", __struct_size(xattr.v->x_name)); > bkey_fsck_err_on(memchr(xattr.v->x_name, '\0', xattr.v->x_name_len), > c, xattr_name_invalid_chars, > "xattr name has invalid characters"); > diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h > index f14c275950b5..43ac0bca485d 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 > > > Here's the generated assembly for this: > > mov rdi, offset .L.str.3 > mov rsi, offset .L__func__.bch2_xattr_validate > mov r12, rdx > mov rdx, -1 > call _printk > mov rax, r12 > movzx esi, ah > movzx edx, byte ptr [r12 + 1] > cmp rsi, rdx > jb .LBB4_15 > # %bb.11: > lea rdi, [rax + 4] > xor ebx, ebx > xor esi, esi > call memchr > > So for the printk it hardcoded -1 (aka 0xFFFFF... 64 bit long int max) > as the result of __struct_size. But then for before call to memchr it does > the same stuff again and puts the second least significant byte of the memory > address of x_type in esi, only to then load the correct value of x_name_len > into edx and compares them for the bounds-check. > __builtin_object_size should only ever be compile time known, right? So it looks like this is pretty broken atm. I think until this stuff is fixed in clang the only real option is: -- diff --git a/include/linux/compiler_attributes.h b/include/linux/compiler_attributes.h index 32284cd26d52..bc5ee8ab4d21 100644 --- a/include/linux/compiler_attributes.h +++ b/include/linux/compiler_attributes.h @@ -101,7 +101,7 @@ * gcc: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108896 * clang: https://github.com/llvm/llvm-project/pull/76348 */ -#if __has_attribute(__counted_by__) +#if __has_attribute(__counted_by__) && !defined(__clang__) # define __counted_by(member) __attribute__((__counted_by__(member))) #else # define __counted_by(member)