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In the Linux kernel, a race condition was discovered in the SMB client’s cached file descriptor (cached_fid) handling. Three status flags—is_open, has_lease, and on_list—were originally stored within the same bitfield byte inside the struct cached_fid. Because C bitfields are packed by the compiler, updating any single flag results in a non-atomic read-modify-write (RMW) operation on the entire byte (e.g., `orb $mask, addr` on x86_64). If two CPU cores attempt to update different flags in this byte simultaneously, one core may read the byte while a second core modifies it, and then the first core writes back its modification based on a stale read, effectively undoing the changes made by the second core. The provided example interleaving shows CPU1 loading a byte where `has_lease` and `on_list` are set, CPU2 clearing that entire byte, and CPU1 then performing an OR operation that incorrectly restores the cleared bits. This can lead to inconsistent state in the SMB client, potentially causing file handle corruption, lease violations, or system instability. The fix implemented in commits like `4386f6af8aaedd0` splits these three flags into separate `bool` fields, ensuring each occupies its own memory location and can be updated atomically without affecting the others.
dailycve form:
Platform: Linux Kernel
Version: 6.6 to 6.19
Vulnerability: Race Condition
Severity: Medium
date: 2026-02-18
Prediction: Already Patched
What Undercode Say:
Analytics:
This vulnerability stems from a common low-level programming pitfall: assuming bitfield operations are atomic. The exploitability relies on precise timing (race window), making it difficult to trigger consistently but possible under high I/O load or on multi-core systems. The SMB client (CIFS) is widely used in enterprise environments for file sharing with Windows and NAS devices, making this a moderate but present risk for server infrastructure. The fix is simple and non-disruptive, demonstrating a shift towards explicit data structures over space-saving micro-optimizations in kernel code.
Bash/Code:
Check your current kernel version
uname -r
On Debian/Ubuntu, check if an update is available
sudo apt update
apt list --upgradable 2>/dev/null | grep -E "linux-image-[0-9]"
On RHEL/CentOS/Fedora
dnf check-update kernel
View the exact code fix (from the mailing list)
The change in fs/smb/client/cached_dir.h would look like:
FROM (Problematic bitfield):
struct cached_fid {
unsigned int is_open:1;
unsigned int has_lease:1;
unsigned int on_list:1;
};
TO (Fixed version):
struct cached_fid {
bool is_open;
bool has_lease;
bool on_list;
};
Simulate the race condition (theoretical C pseudo-code of the bug)
Thread 1:
old_byte = flag_byte; // Reads 0b00000111 (all flags set)
new_byte = old_byte | 0x01; // Sets IS_OPEN (already set)
flag_byte = new_byte; // Writes back 0b00000111
Thread 2 (concurrent):
flag_byte = 0; // Clears all flags to 0
Result: Thread 1's writeback occurs after Thread 2's clear,
resulting in final value 0b00000111 (incorrectly setting cleared flags).
Exploit:
Direct exploitation is challenging due to the narrow race window. An attacker would need local access and the ability to generate concurrent SMB requests targeting the same cached file descriptor. Success could lead to a use-after-free or double-free condition if the flags manage kernel object lifetimes, potentially leading to privilege escalation. However, the primary impact is denial of service through kernel data structure corruption.
Protection:
Protection is achieved by updating to a patched kernel version containing the commits (e.g., 4cfa4c37dcbc). System administrators should apply the latest stable kernel updates for their distribution. As a mitigation, limiting concurrent SMB threads or reducing the caching aggressiveness of the CIFS module (via mount options) might reduce the race window, but patching is the only complete solution.
Impact:
Successful exploitation could cause the SMB client to operate on incorrect file handle states. This can manifest as data corruption, unexpected I/O errors, or kernel panics, leading to denial of service for any application relying on network file shares. Integrity and availability are the primary concerns, as the vulnerability directly affects how the kernel tracks open files and leases.
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Sources:
Reported By: nvd.nist.gov
Extra Source Hub:
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