{"id":"AZL-90497","summary":"CVE-2026-52991 affecting package kernel 6.6.150.1-1","details":"In the Linux kernel, the following vulnerability has been resolved:\n\nsched/psi: fix race between file release and pressure write\n\nA potential race condition exists between pressure write and cgroup file\nrelease regarding the priv member of struct kernfs_open_file, which\ntriggers the uaf reported in [1].\n\nConsider the following scenario involving execution on two separate CPUs:\n\n   CPU0\t\t\t\t\tCPU1\n   ====\t\t\t\t\t====\n\t\t\t\t\tvfs_rmdir()\n\t\t\t\t\tkernfs_iop_rmdir()\n\t\t\t\t\tcgroup_rmdir()\n\t\t\t\t\tcgroup_kn_lock_live()\n\t\t\t\t\tcgroup_destroy_locked()\n\t\t\t\t\tcgroup_addrm_files()\n\t\t\t\t\tcgroup_rm_file()\n\t\t\t\t\tkernfs_remove_by_name()\n\t\t\t\t\tkernfs_remove_by_name_ns()\n vfs_write()\t\t\t\t__kernfs_remove()\n new_sync_write()\t\t\tkernfs_drain()\n kernfs_fop_write_iter()\t\tkernfs_drain_open_files()\n cgroup_file_write()\t\t\tkernfs_release_file()\n pressure_write()\t\t\tcgroup_file_release()\n ctx = of-\u003epriv;\n\t\t\t\t\tkfree(ctx);\n \t\t\t\t\tof-\u003epriv = NULL;\n\t\t\t\t\tcgroup_kn_unlock()\n cgroup_kn_lock_live()\n cgroup_get(cgrp)\n cgroup_kn_unlock()\n if (ctx-\u003epsi.trigger)  // here, trigger uaf for ctx, that is of-\u003epriv\n\nThe cgroup_rmdir() is protected by the cgroup_mutex, it also safeguards\nthe memory deallocation of of-\u003epriv performed within cgroup_file_release().\nHowever, the operations involving of-\u003epriv executed within pressure_write()\nare not entirely covered by the protection of cgroup_mutex. Consequently,\nif the code in pressure_write(), specifically the section handling the\nctx variable executes after cgroup_file_release() has completed, a uaf\nvulnerability involving of-\u003epriv is triggered.\n\nTherefore, the issue can be resolved by extending the scope of the\ncgroup_mutex lock within pressure_write() to encompass all code paths\ninvolving of-\u003epriv, thereby properly synchronizing the race condition\noccurring between cgroup_file_release() and pressure_write().\n\nAnd, if an live kn lock can be successfully acquired while executing\nthe pressure write operation, it indicates that the cgroup deletion\nprocess has not yet reached its final stage; consequently, the priv\npointer within open_file cannot be NULL. Therefore, the operation to\nretrieve the ctx value must be moved to a point *after* the live kn\nlock has been successfully acquired.\n\nIn another situation, specifically after entering cgroup_kn_lock_live()\nbut before acquiring cgroup_mutex, there exists a different class of\nrace condition:\n\nCPU0: write memory.pressure               CPU1: write cgroup.pressure=0\n===========================\t\t  =============================\n\nkernfs_fop_write_iter()\n kernfs_get_active_of(of)\n pressure_write()\n   cgroup_kn_lock_live(memory.pressure)\n     cgroup_tryget(cgrp)\n     kernfs_break_active_protection(kn)\n     ... blocks on cgroup_mutex\n\n                                     \t  cgroup_pressure_write()\n                                     \t  cgroup_kn_lock_live(cgroup.pressure)\n                                     \t  cgroup_file_show(memory.pressure, false)\n                                     \t    kernfs_show(false)\n                                     \t      kernfs_drain_open_files()\n                                     \t        cgroup_file_release(of)\n                                     \t          kfree(ctx)\n                                     \t            of-\u003epriv = NULL\n                                     \t  cgroup_kn_unlock()\n\n   ... acquires cgroup_mutex\n   ctx = of-\u003epriv;        // may now be NULL\n   if (ctx-\u003epsi.trigger)  // NULL dereference\n\nConsequently, there is a possibility that of-\u003epriv is NULL, the pressure\nwrite needs to check for this.\n\nNow that the scope of the cgroup_mutex has been expanded, the original\nexplicit cgroup_get/put operations are no longer necessary, this is\nbecause acquiring/releasing the live kn lock inherently executes a\ncgroup get/put operation.\n\n[1]\nBUG: KASAN: slab-use-after-free in pressure_write+0xa4/0x210 kernel/cgroup/cgroup.c:4011\nCall Trace:\n pressure_write+0xa4/0x210 kernel/cgroup/cgroup.c:4011\n cgroup_file_write+0x36f/0x790 kernel/cgroup/cgroup.c:43\n---truncated---","modified":"2026-09-01T05:28:09Z","published":"2026-06-24T17:17:09Z","upstream":["CVE-2026-52991"],"references":[{"type":"WEB","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-52991"}],"affected":[{"package":{"name":"kernel","ecosystem":"Azure Linux:3","purl":"pkg:rpm/azure-linux/kernel"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"last_affected":"6.6.150.1-1"}]}],"database_specific":{"source":"https://github.com/microsoft/AzureLinuxVulnerabilityData/blob/main/osv/AZL-90497.json"}}],"schema_version":"1.9.0"}