{
  "schema_version": "1.6.1",
  "id": "RPI-CVE-2025-21881",
  "modified": "2026-10-07T17:22:01Z",
  "upstream": [
    "CVE-2025-21881"
  ],
  "published": "2025-03-27T15:01:31Z",
  "summary": "uprobes: Reject the shared zeropage in uprobe_write_opcode()",
  "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   <TASK>\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()->__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->refcount is zero,\n    then free_pages_prepare->free_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    -> ...\n      -> folio_remove_rmap_pte()\n        -> 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 ]",
  "affected": [
    {
      "package": {
        "ecosystem": "Raspberry Pi OS:12",
        "name": "linux"
      },
      "versions": [
        "1:6.1.47-1+rpt4"
      ],
      "ecosystem_specific": {
        "binaries": [
          {
            "binary_name": "linux-image-6.1.0-rpi3-rpi-2712",
            "binary_version": "1:6.1.47-1+rpt4"
          },
          {
            "binary_name": "linux-image-6.1.0-rpi3-rpi-v6",
            "binary_version": "1:6.1.47-1+rpt4"
          },
          {
            "binary_name": "linux-image-6.1.0-rpi3-rpi-v7",
            "binary_version": "1:6.1.47-1+rpt4"
          },
          {
            "binary_name": "linux-image-6.1.0-rpi3-rpi-v7l",
            "binary_version": "1:6.1.47-1+rpt4"
          },
          {
            "binary_name": "linux-image-6.1.0-rpi3-rpi-v8",
            "binary_version": "1:6.1.47-1+rpt4"
          }
        ]
      },
      "database_specific": {
        "per_version": {
          "1:6.1.47-1+rpt4": {
            "linux_commit": "655fc658a15ae7a6f37103754adb39ba52a9a14e",
            "upstream_version": "6.1.47",
            "assessment": "commit ancestry (strak rule)",
            "upstream_fixed": "6.1.130",
            "compiled_filter": false
          }
        }
      }
    }
  ],
  "references": [
    {
      "type": "ADVISORY",
      "url": "https://www.cve.org/CVERecord?id=CVE-2025-21881"
    },
    {
      "type": "FIX",
      "url": "https://git.kernel.org/stable/c/c4cb2bfa99513311886c1eb5c1c2ac26f3338a6e"
    },
    {
      "type": "FIX",
      "url": "https://git.kernel.org/stable/c/0b6f19714588cf2366b0364234f97ba963688f63"
    },
    {
      "type": "FIX",
      "url": "https://git.kernel.org/stable/c/13cca2b73e2b0ec3ea6d6615d615395621d22752"
    },
    {
      "type": "FIX",
      "url": "https://git.kernel.org/stable/c/54011fc94422f094eaf47555284de70a4bc32bb9"
    },
    {
      "type": "FIX",
      "url": "https://git.kernel.org/stable/c/bddf10d26e6e5114e7415a0e442ec6f51a559468"
    }
  ],
  "database_specific": {
    "source": "https://git.kernel.org/pub/scm/linux/security/vulns.git",
    "upstream_modified": "2026-05-12T04:54:46Z"
  }
}