CVE-2024-42136

In the Linux kernel, the following vulnerability has been resolved:

cdrom: rearrange last_media_change check to avoid unintentional overflow

When running syzkaller with the newly reintroduced signed integer wrap sanitizer we encounter this splat:

[ 366.015950] UBSAN: signed-integer-overflow in ../drivers/cdrom/cdrom.c:2361:33 [ 366.021089] -9223372036854775808 - 346321 cannot be represented in type __s64 (aka long long) [ 366.025894] program syz-executor.4 is using a deprecated SCSI ioctl, please convert it to SG_IO [ 366.027502] CPU: 5 PID: 28472 Comm: syz-executor.7 Not tainted 6.8.0-rc2-00035-gb3ef86b5a957 #1 [ 366.027512] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014 [ 366.027518] Call Trace: [ 366.027523] [ 366.027533] dump_stack_lvl+0x93/0xd0 [ 366.027899] handle_overflow+0x171/0x1b0 [ 366.038787] ata1.00: invalid multi_count 32 ignored [ 366.043924] cdrom_ioctl+0x2c3f/0x2d10 [ 366.063932] ? __pm_runtime_resume+0xe6/0x130 [ 366.071923] sr_block_ioctl+0x15d/0x1d0 [ 366.074624] ? __pfx_sr_block_ioctl+0x10/0x10 [ 366.077642] blkdev_ioctl+0x419/0x500 [ 366.080231] ? __pfx_blkdev_ioctl+0x10/0x10 …

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).

Lets rearrange the check to not perform any arithmetic, thus not tripping the sanitizer.

Weakness

The product performs a calculation that can produce an integer overflow or wraparound when the logic assumes that the resulting value will always be larger than the original value. This occurs when an integer value is incremented to a value that is too large to store in the associated representation. When this occurs, the value may become a very small or negative number.

Affected Software

Potential Mitigations

• Use a language that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid.
• If possible, choose a language or compiler that performs automatic bounds checking.
• Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid [REF-1482].
• Use libraries or frameworks that make it easier to handle numbers without unexpected consequences.
• Examples include safe integer handling packages such as SafeInt (C++) or IntegerLib (C or C++). [REF-106]
• Perform input validation on any numeric input by ensuring that it is within the expected range. Enforce that the input meets both the minimum and maximum requirements for the expected range.
• Use unsigned integers where possible. This makes it easier to perform validation for integer overflows. When signed integers are required, ensure that the range check includes minimum values as well as maximum values.
• Understand the programming language’s underlying representation and how it interacts with numeric calculation (CWE-681). Pay close attention to byte size discrepancies, precision, signed/unsigned distinctions, truncation, conversion and casting between types, “not-a-number” calculations, and how the language handles numbers that are too large or too small for its underlying representation. [REF-7]
• Also be careful to account for 32-bit, 64-bit, and other potential differences that may affect the numeric representation.

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/92.html

References

• https://git.kernel.org/stable/c/0c97527e916054acc4a46ffb02842988acb2e92b
• https://git.kernel.org/stable/c/3ee21e14c8c329168a0b66bab00ecd18f5d0dee3
• https://git.kernel.org/stable/c/e809bc112712da8f7e15822674c6562da6cdf24c
• https://git.kernel.org/stable/c/efb905aeb44b0e99c0e6b07865b1885ae0471ebf
• https://git.kernel.org/stable/c/0c97527e916054acc4a46ffb02842988acb2e92b
• https://git.kernel.org/stable/c/3ee21e14c8c329168a0b66bab00ecd18f5d0dee3
• https://git.kernel.org/stable/c/e809bc112712da8f7e15822674c6562da6cdf24c
• https://git.kernel.org/stable/c/efb905aeb44b0e99c0e6b07865b1885ae0471ebf
• https://lists.debian.org/debian-lts-announce/2025/01/msg00001.html