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Mon, 13 May 2024 13:01:58 -0700 (PDT) Received: from www.outflux.net ([198.0.35.241]) by smtp.gmail.com with ESMTPSA id 41be03b00d2f7-6340c4d4a1csm8266124a12.40.2024.05.13.13.01.58 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 13 May 2024 13:01:58 -0700 (PDT) Date: Mon, 13 May 2024 13:01:57 -0700 From: Kees Cook To: Justin Stitt Cc: Alexander Viro , Christian Brauner , Jan Kara , Nathan Chancellor , Bill Wendling , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, llvm@lists.linux.dev, linux-hardening@vger.kernel.org Subject: Re: [PATCH v2] fs: fix unintentional arithmetic wraparound in offset calculation Message-ID: <202405131251.6FD48B6A8@keescook> References: <20240509-b4-sio-read_write-v2-1-018fc1e63392@google.com> Precedence: bulk X-Mailing-List: linux-fsdevel@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: <20240509-b4-sio-read_write-v2-1-018fc1e63392@google.com> On Thu, May 09, 2024 at 11:42:07PM +0000, Justin Stitt wrote: > When running syzkaller with the newly reintroduced signed integer > overflow sanitizer we encounter this report: > > [ 67.995501] UBSAN: signed-integer-overflow in ../fs/read_write.c:91:10 > [ 68.000067] 9223372036854775807 + 4096 cannot be represented in type 'loff_t' (aka 'long long') > [ 68.006266] CPU: 4 PID: 10851 Comm: syz-executor.5 Not tainted 6.8.0-rc2-00035-gb3ef86b5a957 #1 > [ 68.012353] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014 > [ 68.018983] Call Trace: > [ 68.020803] > [ 68.022540] dump_stack_lvl+0x93/0xd0 > [ 68.025222] handle_overflow+0x171/0x1b0 > [ 68.028053] generic_file_llseek_size+0x35b/0x380 > > amongst others: > UBSAN: signed-integer-overflow in ../fs/read_write.c:1657:12 > 142606336 - -9223372036854775807 cannot be represented in type 'loff_t' (aka 'long long') > ... > UBSAN: signed-integer-overflow in ../fs/read_write.c:1666:11 > 9223372036854775807 - -9223231299366420479 cannot be represented in type 'loff_t' (aka 'long long') > > Historically, the signed integer overflow sanitizer did not work in the > kernel due to its interaction with `-fwrapv` but this has since been > changed [1] in the newest version of Clang. It was re-enabled in the > kernel with Commit 557f8c582a9ba8ab ("ubsan: Reintroduce signed overflow > sanitizer"). > > Fix the accidental overflow in these position and offset calculations > by checking for negative position values, using check_add_overflow() > helpers and clamping values to expected ranges. > > Since @offset is later limited by @maxsize, we can proactively safeguard > against exceeding that value (and by extension avoiding integer overflow): > loff_t vfs_setpos(struct file *file, loff_t offset, loff_t maxsize) > { > if (offset < 0 && !unsigned_offsets(file)) > return -EINVAL; > if (offset > maxsize) > return -EINVAL; > ... > > Link: https://github.com/llvm/llvm-project/pull/82432 [1] > Closes: https://github.com/KSPP/linux/issues/358 > Cc: linux-hardening@vger.kernel.org > Signed-off-by: Justin Stitt > --- > Changes in v2: > - fix some more cases syzkaller found in read_write.c > - use min over min_t as the types are the same > - Link to v1: https://lore.kernel.org/r/20240509-b4-sio-read_write-v1-1-06bec2022697@google.com > --- > Here's the syzkaller reproducer: > | # {Threaded:false Repeat:false RepeatTimes:0 Procs:1 Slowdown:1 Sandbox: > | # SandboxArg:0 Leak:false NetInjection:false NetDevices:false > | # NetReset:false Cgroups:false BinfmtMisc:false CloseFDs:false KCSAN:false > | # DevlinkPCI:false NicVF:false USB:false VhciInjection:false Wifi:false > | # IEEE802154:false Sysctl:false Swap:false UseTmpDir:false > | # HandleSegv:false Repro:false Trace:false LegacyOptions:{Collide:false > | # Fault:false FaultCall:0 FaultNth:0}} > | r0 = openat$sysfs(0xffffffffffffff9c, &(0x7f0000000000)='/sys/kernel/address_bits', 0x0, 0x98) > | lseek(r0, 0x7fffffffffffffff, 0x2) > > ... which was used against Kees' tree here (v6.8rc2): > https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git/log/?h=wip/v6.9-rc2/unsigned-overflow-sanitizer > > ... with this config: > https://gist.github.com/JustinStitt/824976568b0f228ccbcbe49f3dee9bf4 > --- > fs/read_write.c | 18 +++++++++++------- > fs/remap_range.c | 12 ++++++------ > 2 files changed, 17 insertions(+), 13 deletions(-) > > diff --git a/fs/read_write.c b/fs/read_write.c > index d4c036e82b6c..d116e6e3eb3d 100644 > --- a/fs/read_write.c > +++ b/fs/read_write.c > @@ -88,7 +88,7 @@ generic_file_llseek_size(struct file *file, loff_t offset, int whence, > { > switch (whence) { > case SEEK_END: > - offset += eof; > + offset = min(offset, maxsize - eof) + eof; This seems effectively unchanged compared to v1? https://lore.kernel.org/all/CAFhGd8qbUYXmgiFuLGQ7dWXFUtZacvT82wD4jSS-xNTvtzXKGQ@mail.gmail.com/ > break; > case SEEK_CUR: > /* > @@ -105,7 +105,8 @@ generic_file_llseek_size(struct file *file, loff_t offset, int whence, > * like SEEK_SET. > */ > spin_lock(&file->f_lock); > - offset = vfs_setpos(file, file->f_pos + offset, maxsize); > + offset = vfs_setpos(file, min(file->f_pos, maxsize - offset) + > + offset, maxsize); > spin_unlock(&file->f_lock); > return offset; > case SEEK_DATA: > @@ -1416,7 +1417,7 @@ static int generic_copy_file_checks(struct file *file_in, loff_t pos_in, > struct inode *inode_in = file_inode(file_in); > struct inode *inode_out = file_inode(file_out); > uint64_t count = *req_count; > - loff_t size_in; > + loff_t size_in, in_sum, out_sum; > int ret; > > ret = generic_file_rw_checks(file_in, file_out); > @@ -1450,8 +1451,8 @@ static int generic_copy_file_checks(struct file *file_in, loff_t pos_in, > if (IS_SWAPFILE(inode_in) || IS_SWAPFILE(inode_out)) > return -ETXTBSY; > > - /* Ensure offsets don't wrap. */ > - if (pos_in + count < pos_in || pos_out + count < pos_out) > + if (check_add_overflow(pos_in, count, &in_sum) || > + check_add_overflow(pos_out, count, &out_sum)) > return -EOVERFLOW; I like these changes -- they make this much more readable. > > /* Shorten the copy to EOF */ > @@ -1467,8 +1468,8 @@ static int generic_copy_file_checks(struct file *file_in, loff_t pos_in, > > /* Don't allow overlapped copying within the same file. */ > if (inode_in == inode_out && > - pos_out + count > pos_in && > - pos_out < pos_in + count) > + out_sum > pos_in && > + pos_out < in_sum) > return -EINVAL; > > *req_count = count; > @@ -1649,6 +1650,9 @@ int generic_write_check_limits(struct file *file, loff_t pos, loff_t *count) > loff_t max_size = inode->i_sb->s_maxbytes; > loff_t limit = rlimit(RLIMIT_FSIZE); > > + if (pos < 0) > + return -EINVAL; > + > if (limit != RLIM_INFINITY) { > if (pos >= limit) { > send_sig(SIGXFSZ, current, 0); > diff --git a/fs/remap_range.c b/fs/remap_range.c > index de07f978ce3e..4570be4ef463 100644 > --- a/fs/remap_range.c > +++ b/fs/remap_range.c > @@ -36,7 +36,7 @@ static int generic_remap_checks(struct file *file_in, loff_t pos_in, > struct inode *inode_out = file_out->f_mapping->host; > uint64_t count = *req_count; > uint64_t bcount; > - loff_t size_in, size_out; > + loff_t size_in, size_out, in_sum, out_sum; > loff_t bs = inode_out->i_sb->s_blocksize; > int ret; > > @@ -44,17 +44,17 @@ static int generic_remap_checks(struct file *file_in, loff_t pos_in, > if (!IS_ALIGNED(pos_in, bs) || !IS_ALIGNED(pos_out, bs)) > return -EINVAL; > > - /* Ensure offsets don't wrap. */ > - if (pos_in + count < pos_in || pos_out + count < pos_out) > - return -EINVAL; > + if (check_add_overflow(pos_in, count, &in_sum) || > + check_add_overflow(pos_out, count, &out_sum)) > + return -EOVERFLOW; Yeah, this is a good error code change. This is ultimately exposed via copy_file_range, where this error is documented as possible. -Kees -- Kees Cook