Bug 2419870 (CVE-2025-40304)

Summary: CVE-2025-40304 kernel: Linux kernel: Out-of-bounds write in fbdev can lead to privilege escalation, information disclosure, or denial of service.
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
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Version: unspecifiedKeywords: Security
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OS: Linux   
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A flaw was found in the Linux kernel, specifically within the framebuffer device (fbdev) subsystem. This vulnerability, an out-of-bounds write, occurs because the `bit_putcs` function does not properly check boundaries when displaying text near the edges of the screen. A local user with low privileges could exploit this to write data beyond the intended memory area. This could potentially lead to serious consequences such as gaining unauthorized access (privilege escalation), revealing sensitive information (information disclosure), or causing the system to become unavailable (denial of service).
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Description OSIDB Bzimport 2025-12-08 07:04:51 UTC 
In the Linux kernel, the following vulnerability has been resolved:

fbdev: Add bounds checking in bit_putcs to fix vmalloc-out-of-bounds

Add bounds checking to prevent writes past framebuffer boundaries when
rendering text near screen edges. Return early if the Y position is off-screen
and clip image height to screen boundary. Break from the rendering loop if the
X position is off-screen. When clipping image width to fit the screen, update
the character count to match the clipped width to prevent buffer size
mismatches.

Without the character count update, bit_putcs_aligned and bit_putcs_unaligned
receive mismatched parameters where the buffer is allocated for the clipped
width but cnt reflects the original larger count, causing out-of-bounds writes.
Comment 1 Jon Moroney 2025-12-08 23:42:28 UTC 
Upstream advisory:
https://lore.kernel.org/linux-cve-announce/2025120820-CVE-2025-40304-47b3@gregkh/T
Comment 4 errata-xmlrpc 2026-02-02 15:25:22 UTC 
This issue has been addressed in the following products:

  Red Hat Enterprise Linux 10.0 Extended Update Support

Via RHSA-2026:1727 https://access.redhat.com/errata/RHSA-2026:1727