CVE-2024-41003 in Linux
Summary
by MITRE • 07/12/2024
In the Linux kernel, the following vulnerability has been resolved:
bpf: Fix reg_set_min_max corruption of fake_reg
Juan reported that after doing some changes to buzzer [0] and implementing
a new fuzzing strategy guided by coverage, they noticed the following in one of the probes:
[...]
13: (79) r6 = *(u64 *)(r0 +0) ; R0=map_value(ks=4,vs=8) R6_w=scalar() 14: (b7) r0 = 0 ; R0_w=0 15: (b4) w0 = -1 ; R0_w=0xffffffff 16: (74) w0 >>= 1 ; R0_w=0x7fffffff 17: (5c) w6 &= w0 ; R0_w=0x7fffffff R6_w=scalar(smin=smin32=0,smax=umax=umax32=0x7fffffff,var_off=(0x0; 0x7fffffff)) 18: (44) w6 |= 2 ; R6_w=scalar(smin=umin=smin32=umin32=2,smax=umax=umax32=0x7fffffff,var_off=(0x2; 0x7ffffffd)) 19: (56) if w6 != 0x7ffffffd goto pc+1 REG INVARIANTS VIOLATION (true_reg2): range bounds violation u64=[0x7fffffff, 0x7ffffffd] s64=[0x7fffffff, 0x7ffffffd] u32=[0x7fffffff, 0x7ffffffd] s32=[0x7fffffff, 0x7ffffffd] var_off=(0x7fffffff, 0x0)
REG INVARIANTS VIOLATION (false_reg1): range bounds violation u64=[0x7fffffff, 0x7ffffffd] s64=[0x7fffffff, 0x7ffffffd] u32=[0x7fffffff, 0x7ffffffd] s32=[0x7fffffff, 0x7ffffffd] var_off=(0x7fffffff, 0x0)
REG INVARIANTS VIOLATION (false_reg2): const tnum out of sync with range bounds u64=[0x0, 0xffffffffffffffff] s64=[0x8000000000000000, 0x7fffffffffffffff] u32=[0x0, 0xffffffff] s32=[0x80000000, 0x7fffffff] var_off=(0x7fffffff, 0x0)
19: R6_w=0x7fffffff 20: (95) exit
from 19 to 21: R0=0x7fffffff R6=scalar(smin=umin=smin32=umin32=2,smax=umax=smax32=umax32=0x7ffffffe,var_off=(0x2; 0x7ffffffd)) R7=map_ptr(ks=4,vs=8) R9=ctx() R10=fp0 fp-24=map_ptr(ks=4,vs=8) fp-40=mmmmmmmm 21: R0=0x7fffffff R6=scalar(smin=umin=smin32=umin32=2,smax=umax=smax32=umax32=0x7ffffffe,var_off=(0x2; 0x7ffffffd)) R7=map_ptr(ks=4,vs=8) R9=ctx() R10=fp0 fp-24=map_ptr(ks=4,vs=8) fp-40=mmmmmmmm 21: (14) w6 -= 2147483632 ; R6_w=scalar(smin=umin=umin32=2,smax=umax=0xffffffff,smin32=0x80000012,smax32=14,var_off=(0x2; 0xfffffffd)) 22: (76) if w6 s>= 0xe goto pc+1 ; R6_w=scalar(smin=umin=umin32=2,smax=umax=0xffffffff,smin32=0x80000012,smax32=13,var_off=(0x2; 0xfffffffd)) 23: (95) exit
from 22 to 24: R0=0x7fffffff R6_w=14 R7=map_ptr(ks=4,vs=8) R9=ctx() R10=fp0 fp-24=map_ptr(ks=4,vs=8) fp-40=mmmmmmmm 24: R0=0x7fffffff R6_w=14 R7=map_ptr(ks=4,vs=8) R9=ctx() R10=fp0 fp-24=map_ptr(ks=4,vs=8) fp-40=mmmmmmmm 24: (14) w6 -= 14 ; R6_w=0 [...]
What can be seen here is a register invariant violation on line 19. After the binary-or in line 18, the verifier knows that bit 2 is set but knows nothing about the rest of the content which was loaded from a map value, meaning, range is [2,0x7fffffff] with var_off=(0x2; 0x7ffffffd). When in
line 19 the verifier analyzes the branch, it splits the register states in reg_set_min_max() into the registers of the true branch (true_reg1, true_reg2) and the registers of the false branch (false_reg1, false_reg2).
Since the test is w6 != 0x7ffffffd, the src_reg is a known constant. Internally, the verifier creates a "fake" register initialized as scalar to the value of 0x7ffffffd, and then passes it onto reg_set_min_max(). Now, for line 19, it is mathematically impossible to take the false branch of this program, yet the verifier analyzes it. It is impossible because the second bit of r6 will be set due to the prior or operation and the constant in the condition has that bit unset (hex(fd) == binary(1111 1101).
When the verifier first analyzes the false / fall-through branch, it will compute an intersection between the var_off of r6 and of the constant. This is because the verifier creates a "fake" register initialized to the value of the constant. The intersection result later refines both registers in regs_refine_cond_op():
[...]
t = tnum_intersect(tnum_subreg(reg1->var_off), tnum_subreg(reg2->var_off)); reg1->var_o ---truncated---
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Analysis
by VulDB Data Team • 09/25/2025
The vulnerability described in CVE-2024-41003 resides within the Linux kernel's eBPF (extended Berkeley Packet Filter) verifier subsystem, specifically in how it handles register state management during conditional branch analysis. This flaw manifests as a corruption of register min-max values through improper handling of fake register states during conditional operations. The issue stems from a failure in the `reg_set_min_max` function when processing branch conditions involving scalar values with known bit patterns, leading to incorrect register invariant validation and potential exploitation. The vulnerability is classified under CWE-682, which deals with incorrect use of mathematical operations, and aligns with ATT&CK technique T1059.006 for execution through scripting and T1548.001 for privilege escalation via kernel exploits.
The technical root cause occurs when the eBPF verifier processes a conditional branch where a register value is compared against a constant. In this specific case, after performing a bitwise OR operation that sets bit 2 of the register, the verifier creates a "fake" register initialized to the constant value of 0x7fffffff. When analyzing the false branch of the comparison, the verifier attempts to intersect the variable offset (var_off) of the original register with that of the fake register. This intersection operation incorrectly modifies the register's min-max bounds, creating a scenario where the register's valid range becomes mathematically inconsistent. The inconsistency arises because the original register's value is constrained to have bit 2 set due to the OR operation, while the constant used in the comparison has bit 2 unset, making the false branch logically impossible yet still processed by the verifier.
The operational impact of this vulnerability is significant as it can lead to incorrect verification of eBPF programs, potentially allowing malicious code to bypass security checks and execute arbitrary operations with elevated privileges. An attacker could craft an eBPF program that exploits this register corruption to manipulate kernel memory or bypass security controls. The vulnerability affects systems running Linux kernels with eBPF support, particularly those utilizing eBPF for security monitoring, network filtering, or system call tracing. The issue is particularly concerning because it operates at the kernel level, where incorrect register handling can result in privilege escalation, memory corruption, or denial of service conditions that could be leveraged by attackers with limited access to execute arbitrary code with kernel privileges.
Mitigation strategies include applying the latest kernel updates that contain the patched verifier logic, which properly handles the intersection of variable offsets during conditional branch analysis. Administrators should also implement strict eBPF program validation policies, limit eBPF program capabilities where possible, and monitor for unusual system behavior that might indicate exploitation attempts. Additionally, organizations should consider implementing kernel lockdown modes and other security hardening measures that restrict eBPF program execution to trusted sources. The fix ensures that when fake registers are created for conditional operations, their variable offset handling does not corrupt the actual register state, thereby maintaining the integrity of the verifier's range analysis and preventing incorrect program validation that could lead to privilege escalation or system compromise.