Bug 2419912 (CVE-2025-40287)

Summary: CVE-2025-40287 kernel: exfat: fix improper check of dentry.stream.valid_size
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
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Priority: medium    
Version: unspecifiedKeywords: Security
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OS: Linux   
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An infinite loop vulnerability was found in the exFAT filesystem driver in the Linux kernel. The exfat_find() function does not validate if dentry.stream.valid_size is negative. A malformed exFAT filesystem with such entries causes the kernel to hang during file operations like open, truncate, or write.
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Description OSIDB Bzimport 2025-12-08 07:08:20 UTC 
In the Linux kernel, the following vulnerability has been resolved:

exfat: fix improper check of dentry.stream.valid_size

We found an infinite loop bug in the exFAT file system that can lead to a
Denial-of-Service (DoS) condition. When a dentry in an exFAT filesystem is
malformed, the following system calls — SYS_openat, SYS_ftruncate, and
SYS_pwrite64 — can cause the kernel to hang.

Root cause analysis shows that the size validation code in exfat_find()
does not check whether dentry.stream.valid_size is negative. As a result,
the system calls mentioned above can succeed and eventually trigger the DoS
issue.

This patch adds a check for negative dentry.stream.valid_size to prevent
this vulnerability.
Comment 1 Jon Moroney 2025-12-08 20:20:44 UTC 
Upstream advisory:
https://lore.kernel.org/linux-cve-announce/2025120719-CVE-2025-40287-a68f@gregkh/T