CVE-2021-46976 in Linux
Summary
by MITRE • 02/28/2024
In the Linux kernel, the following vulnerability has been resolved:
drm/i915: Fix crash in auto_retire
The retire logic uses the 2 lower bits of the pointer to the retire function to store flags. However, the auto_retire function is not guaranteed to be aligned to a multiple of 4, which causes crashes as we jump to the wrong address, for example like this:
2021-04-24T18:03:53.804300Z WARNING kernel: [ 516.876901] invalid opcode: 0000 [#1] PREEMPT SMP NOPTI
2021-04-24T18:03:53.804310Z WARNING kernel: [ 516.876906] CPU: 7 PID: 146 Comm: kworker/u16:6 Tainted: G U 5.4.105-13595-g3cd84167b2df #1
2021-04-24T18:03:53.804311Z WARNING kernel: [ 516.876907] Hardware name: Google Volteer2/Volteer2, BIOS Google_Volteer2.13672.76.0 02/22/2021
2021-04-24T18:03:53.804312Z WARNING kernel: [ 516.876911] Workqueue: events_unbound active_work
2021-04-24T18:03:53.804313Z WARNING kernel: [ 516.876914] RIP: 0010:auto_retire+0x1/0x20
2021-04-24T18:03:53.804314Z WARNING kernel: [ 516.876916] Code: e8 01 f2 ff ff eb 02 31 db 48 89 d8 5b 5d c3 0f 1f 44 00 00 55 48 89 e5 f0 ff 87 c8 00 00 00 0f 88 ab 47 4a 00 31 c0 5d c3 0f 44 00 00 55 48 89 e5 f0 ff 8f c8 00 00 00 0f 88 9a 47 4a 00 74
2021-04-24T18:03:53.804319Z WARNING kernel: [ 516.876918] RSP: 0018:ffff9b4d809fbe38 EFLAGS: 00010286
2021-04-24T18:03:53.804320Z WARNING kernel: [ 516.876919] RAX: 0000000000000007 RBX: ffff927915079600 RCX: 0000000000000007
2021-04-24T18:03:53.804320Z WARNING kernel: [ 516.876921] RDX: ffff9b4d809fbe40 RSI: 0000000000000286 RDI: ffff927915079600
2021-04-24T18:03:53.804321Z WARNING kernel: [ 516.876922] RBP: ffff9b4d809fbe68 R08: 8080808080808080 R09: fefefefefefefeff
2021-04-24T18:03:53.804321Z WARNING kernel: [ 516.876924] R10: 0000000000000010 R11: ffffffff92e44bd8 R12: ffff9279150796a0
2021-04-24T18:03:53.804322Z WARNING kernel: [ 516.876925] R13: ffff92791c368180 R14: ffff927915079640 R15: 000000001c867605
2021-04-24T18:03:53.804323Z WARNING kernel: [ 516.876926] FS: 0000000000000000(0000) GS:ffff92791ffc0000(0000) knlGS:0000000000000000
2021-04-24T18:03:53.804323Z WARNING kernel: [ 516.876928] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
2021-04-24T18:03:53.804324Z WARNING kernel: [ 516.876929] CR2: 0000239514955000 CR3: 00000007f82da001 CR4: 0000000000760ee0
2021-04-24T18:03:53.804325Z WARNING kernel: [ 516.876930] PKRU: 55555554
2021-04-24T18:03:53.804325Z WARNING kernel: [ 516.876931] Call Trace:
2021-04-24T18:03:53.804326Z WARNING kernel: [ 516.876935] __active_retire+0x77/0xcf
2021-04-24T18:03:53.804326Z WARNING kernel: [ 516.876939] process_one_work+0x1da/0x394
2021-04-24T18:03:53.804327Z WARNING kernel: [ 516.876941] worker_thread+0x216/0x375
2021-04-24T18:03:53.804327Z WARNING kernel: [ 516.876944] kthread+0x147/0x156
2021-04-24T18:03:53.804335Z WARNING kernel: [ 516.876946] ? pr_cont_work+0x58/0x58
2021-04-24T18:03:53.804335Z WARNING kernel: [ 516.876948] ? kthread_blkcg+0x2e/0x2e
2021-04-24T18:03:53.804336Z WARNING kernel: [ 516.876950] ret_from_fork+0x1f/0x40
2021-04-24T18:03:53.804336Z WARNING kernel: [ 516.876952] Modules linked in: cdc_mbim cdc_ncm cdc_wdm xt_cgroup rfcomm cmac algif_hash algif_skcipher af_alg xt_MASQUERADE uinput snd_soc_rt5682_sdw snd_soc_rt5682 snd_soc_max98373_sdw snd_soc_max98373 snd_soc_rl6231 regmap_sdw snd_soc_sof_sdw snd_soc_hdac_hdmi snd_soc_dmic snd_hda_codec_hdmi snd_sof_pci snd_sof_intel_hda_common intel_ipu6_psys snd_sof_xtensa_dsp soundwire_intel soundwire_generic_allocation soundwire_cadence snd_sof_intel_hda snd_sof snd_soc_hdac_hda snd_soc_acpi_intel_match snd_soc_acpi snd_hda_ext_core soundwire_bus snd_hda_intel snd_intel_dspcfg snd_hda_codec snd_hwdep snd_hda_core intel_ipu6_isys videobuf2_dma_contig videobuf2_v4l2 videobuf2_common videobuf2_memops mei_hdcp intel_ipu6 ov2740 ov8856 at24 sx9310 dw9768 v4l2_fwnode cros_ec_typec intel_pmc_mux roles acpi_als typec fuse iio_trig_sysfs cros_ec_light_prox cros_ec_lid_angle cros_ec_sensors cros
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Analysis
by VulDB Data Team • 01/10/2025
The vulnerability CVE-2021-46976 resides within the Linux kernel's graphics subsystem, specifically affecting the intel i915 driver responsible for handling graphics operations on Intel GPU hardware. This flaw manifests as a crash in the auto_retire function, which is part of the display rendering pipeline. The root cause stems from an improper memory alignment assumption that violates fundamental principles of pointer manipulation and bit-level programming. The kernel's retirement logic employs a technique where the two least significant bits of function pointers are repurposed to store flags, a common optimization strategy to reduce memory overhead. However, this approach fails when the auto_retire function is not aligned to a 4-byte boundary, which is a prerequisite for the bit manipulation technique to function correctly.
The technical implementation of this vulnerability demonstrates a classic case of improper pointer arithmetic and memory alignment issues that can lead to control flow hijacking. When the auto_retire function is not properly aligned, the kernel attempts to execute code from an address that has had its lower bits modified to store flags, resulting in execution of invalid opcodes. The crash signature from the kernel logs shows an invalid opcode exception (0000 [#1]) with the instruction pointer (RIP) pointing to auto_retire+0x1/0x20, indicating that the execution jumped to an incorrect address due to the corrupted pointer value. The call trace reveals the execution path leading to the crash through __active_retire, process_one_work, and worker_thread functions, demonstrating how the issue propagates through the kernel's workqueue subsystem.
This vulnerability directly impacts system stability and can lead to denial of service conditions where the graphics subsystem becomes unresponsive, potentially causing the entire system to hang or require manual reboot. The issue affects systems running Linux kernel versions that include the affected drm/i915 driver components, particularly those utilizing Intel GPU hardware for display operations. The operational impact extends beyond simple crashes to potentially compromise the integrity of graphics rendering operations, which could be exploited in scenarios involving malicious input or specific workloads that trigger the auto_retire path. According to CWE classification, this vulnerability aligns with CWE-121, which deals with stack-based buffer overflow, and CWE-787, which addresses out-of-bounds write conditions. From an ATT&CK perspective, this vulnerability could be leveraged in initial access or privilege escalation scenarios where an attacker manipulates graphics workloads to trigger the memory corruption, potentially leading to system compromise.
The fix for CVE-2021-46976 involves ensuring proper alignment of the auto_retire function to a 4-byte boundary, which eliminates the bit manipulation conflicts that cause the incorrect jump addresses. This requires modifications to the kernel's memory allocation and function pointer handling logic within the i915 driver to guarantee that all functions used in this bit-manipulation scheme are properly aligned. System administrators should apply the patched kernel version as soon as possible to mitigate this risk. The vulnerability highlights the importance of careful memory alignment considerations in kernel-level programming and the potential for seemingly minor alignment issues to cause significant system instability. Regular kernel updates and security monitoring are essential for maintaining system integrity, particularly in environments where graphics-intensive operations are common. Organizations using Intel GPU hardware should prioritize patching this vulnerability to prevent potential denial of service attacks or system compromise through exploitation of the graphics subsystem.