{"id":"AZL-69494","summary":"CVE-2025-21881 affecting package kernel 5.15.200.1-1","details":"In the Linux kernel, the following vulnerability has been resolved:\n\nuprobes: Reject the shared zeropage in uprobe_write_opcode()\n\nWe triggered the following crash in syzkaller tests:\n\n  BUG: Bad page state in process syz.7.38  pfn:1eff3\n  page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x1eff3\n  flags: 0x3fffff00004004(referenced|reserved|node=0|zone=1|lastcpupid=0x1fffff)\n  raw: 003fffff00004004 ffffe6c6c07bfcc8 ffffe6c6c07bfcc8 0000000000000000\n  raw: 0000000000000000 0000000000000000 00000000fffffffe 0000000000000000\n  page dumped because: PAGE_FLAGS_CHECK_AT_FREE flag(s) set\n  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1ubuntu1.1 04/01/2014\n  Call Trace:\n   \u003cTASK\u003e\n   dump_stack_lvl+0x32/0x50\n   bad_page+0x69/0xf0\n   free_unref_page_prepare+0x401/0x500\n   free_unref_page+0x6d/0x1b0\n   uprobe_write_opcode+0x460/0x8e0\n   install_breakpoint.part.0+0x51/0x80\n   register_for_each_vma+0x1d9/0x2b0\n   __uprobe_register+0x245/0x300\n   bpf_uprobe_multi_link_attach+0x29b/0x4f0\n   link_create+0x1e2/0x280\n   __sys_bpf+0x75f/0xac0\n   __x64_sys_bpf+0x1a/0x30\n   do_syscall_64+0x56/0x100\n   entry_SYSCALL_64_after_hwframe+0x78/0xe2\n\n   BUG: Bad rss-counter state mm:00000000452453e0 type:MM_FILEPAGES val:-1\n\nThe following syzkaller test case can be used to reproduce:\n\n  r2 = creat(&(0x7f0000000000)='./file0\\x00', 0x8)\n  write$nbd(r2, &(0x7f0000000580)=ANY=[], 0x10)\n  r4 = openat(0xffffffffffffff9c, &(0x7f0000000040)='./file0\\x00', 0x42, 0x0)\n  mmap$IORING_OFF_SQ_RING(&(0x7f0000ffd000/0x3000)=nil, 0x3000, 0x0, 0x12, r4, 0x0)\n  r5 = userfaultfd(0x80801)\n  ioctl$UFFDIO_API(r5, 0xc018aa3f, &(0x7f0000000040)={0xaa, 0x20})\n  r6 = userfaultfd(0x80801)\n  ioctl$UFFDIO_API(r6, 0xc018aa3f, &(0x7f0000000140))\n  ioctl$UFFDIO_REGISTER(r6, 0xc020aa00, &(0x7f0000000100)={{&(0x7f0000ffc000/0x4000)=nil, 0x4000}, 0x2})\n  ioctl$UFFDIO_ZEROPAGE(r5, 0xc020aa04, &(0x7f0000000000)={{&(0x7f0000ffd000/0x1000)=nil, 0x1000}})\n  r7 = bpf$PROG_LOAD(0x5, &(0x7f0000000140)={0x2, 0x3, &(0x7f0000000200)=ANY=[@ANYBLOB=\"1800000000120000000000000000000095\"], &(0x7f0000000000)='GPL\\x00', 0x7, 0x0, 0x0, 0x0, 0x0, '\\x00', 0x0, @fallback=0x30, 0xffffffffffffffff, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x10, 0x0, @void, @value}, 0x94)\n  bpf$BPF_LINK_CREATE_XDP(0x1c, &(0x7f0000000040)={r7, 0x0, 0x30, 0x1e, @val=@uprobe_multi={&(0x7f0000000080)='./file0\\x00', &(0x7f0000000100)=[0x2], 0x0, 0x0, 0x1}}, 0x40)\n\nThe cause is that zero pfn is set to the PTE without increasing the RSS\ncount in mfill_atomic_pte_zeropage() and the refcount of zero folio does\nnot increase accordingly. Then, the operation on the same pfn is performed\nin uprobe_write_opcode()-\u003e__replace_page() to unconditional decrease the\nRSS count and old_folio's refcount.\n\nTherefore, two bugs are introduced:\n\n 1. The RSS count is incorrect, when process exit, the check_mm() report\n    error \"Bad rss-count\".\n\n 2. The reserved folio (zero folio) is freed when folio-\u003erefcount is zero,\n    then free_pages_prepare-\u003efree_page_is_bad() report error\n    \"Bad page state\".\n\nThere is more, the following warning could also theoretically be triggered:\n\n  __replace_page()\n    -\u003e ...\n      -\u003e folio_remove_rmap_pte()\n        -\u003e VM_WARN_ON_FOLIO(is_zero_folio(folio), folio)\n\nConsidering that uprobe hit on the zero folio is a very rare case, just\nreject zero old folio immediately after get_user_page_vma_remote().\n\n[ mingo: Cleaned up the changelog ]","modified":"2026-04-01T05:21:34.868677Z","published":"2025-03-27T15:15:56Z","upstream":["CVE-2025-21881"],"references":[{"type":"WEB","url":"https://nvd.nist.gov/vuln/detail/CVE-2025-21881"}],"affected":[{"package":{"name":"kernel","ecosystem":"Azure Linux:2","purl":"pkg:rpm/azure-linux/kernel"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"last_affected":"5.15.200.1-1"}]}],"database_specific":{"source":"https://github.com/microsoft/AzureLinuxVulnerabilityData/blob/main/osv/AZL-69494.json"}}],"schema_version":"1.7.5"}