CVE-2026-43492

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

lib/crypto: mpi: Fix integer underflow in mpi_read_raw_from_sgl()

Yiming reports an integer underflow in mpi_read_raw_from_sgl() when subtracting lzeros from the unsigned nbytes.

For this to happen, the scatterlist sgl needs to occupy more bytes than the nbytes parameter and the first nbytes + 1 bytes of the scatterlist must be zero. Under these conditions, the while loop iterating over the scatterlist will count more zeroes than nbytes, subtract the number of zeroes from nbytes and cause the underflow.

When commit 2d4d1eea540b (lib/mpi: Add mpi sgl helpers) originally introduced the bug, it couldnt be triggered because all callers of mpi_read_raw_from_sgl() passed a scatterlist whose length was equal to nbytes.

However since commit 63ba4d67594a (KEYS: asymmetric: Use new crypto interface without scatterlists), the underflow can now actually be triggered. When invoking a KEYCTL_PKEY_ENCRYPT system call with a larger out_len than in_len and filling the in buffer with zeroes, crypto_akcipher_sync_prep() will create an all-zero scatterlist used for both the src and dst member of struct akcipher_request and thereby fulfil the conditions to trigger the bug:

sys_keyctl() keyctl_pkey_e_d_s() asymmetric_key_eds_op() software_key_eds_op() crypto_akcipher_sync_encrypt() crypto_akcipher_sync_prep() crypto_akcipher_encrypt() rsa_enc() mpi_read_raw_from_sgl()

To the user this will be visible as a DoS as the kernel spins forever, causing soft lockup splats as a side effect.

Fix it.

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/1abd50fc3cfa16fc2074a3c8c2729c50fd9d7043
• https://git.kernel.org/stable/c/26d3a97ad46c7a9226ec04d4bf35bd4998a97d16
• https://git.kernel.org/stable/c/2aa77a18dc7f2670497fe3ee5acbeda0b57659e5
• https://git.kernel.org/stable/c/30e513e755bb381afce6fb57cdc8694136193f22
• https://git.kernel.org/stable/c/6d63615c796c085b4984e3031b9fa77fffb47360
• https://git.kernel.org/stable/c/8637dfb4c1d8a7026ef681f2477c6de8b71c4003
• https://git.kernel.org/stable/c/8c2f1288250a90a4b5cabed5d888d7e3aeed4035
• https://git.kernel.org/stable/c/a1793a48881ec8d26e9c79b0ee65753f1c6846a4