CVE-2025-21881
Publication date:
27/03/2025
In the Linux kernel, the following vulnerability has been resolved:<br />
<br />
uprobes: Reject the shared zeropage in uprobe_write_opcode()<br />
<br />
We triggered the following crash in syzkaller tests:<br />
<br />
BUG: Bad page state in process syz.7.38 pfn:1eff3<br />
page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x1eff3<br />
flags: 0x3fffff00004004(referenced|reserved|node=0|zone=1|lastcpupid=0x1fffff)<br />
raw: 003fffff00004004 ffffe6c6c07bfcc8 ffffe6c6c07bfcc8 0000000000000000<br />
raw: 0000000000000000 0000000000000000 00000000fffffffe 0000000000000000<br />
page dumped because: PAGE_FLAGS_CHECK_AT_FREE flag(s) set<br />
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1ubuntu1.1 04/01/2014<br />
Call Trace:<br />
<br />
dump_stack_lvl+0x32/0x50<br />
bad_page+0x69/0xf0<br />
free_unref_page_prepare+0x401/0x500<br />
free_unref_page+0x6d/0x1b0<br />
uprobe_write_opcode+0x460/0x8e0<br />
install_breakpoint.part.0+0x51/0x80<br />
register_for_each_vma+0x1d9/0x2b0<br />
__uprobe_register+0x245/0x300<br />
bpf_uprobe_multi_link_attach+0x29b/0x4f0<br />
link_create+0x1e2/0x280<br />
__sys_bpf+0x75f/0xac0<br />
__x64_sys_bpf+0x1a/0x30<br />
do_syscall_64+0x56/0x100<br />
entry_SYSCALL_64_after_hwframe+0x78/0xe2<br />
<br />
BUG: Bad rss-counter state mm:00000000452453e0 type:MM_FILEPAGES val:-1<br />
<br />
The following syzkaller test case can be used to reproduce:<br />
<br />
r2 = creat(&(0x7f0000000000)=&#39;./file0\x00&#39;, 0x8)<br />
write$nbd(r2, &(0x7f0000000580)=ANY=[], 0x10)<br />
r4 = openat(0xffffffffffffff9c, &(0x7f0000000040)=&#39;./file0\x00&#39;, 0x42, 0x0)<br />
mmap$IORING_OFF_SQ_RING(&(0x7f0000ffd000/0x3000)=nil, 0x3000, 0x0, 0x12, r4, 0x0)<br />
r5 = userfaultfd(0x80801)<br />
ioctl$UFFDIO_API(r5, 0xc018aa3f, &(0x7f0000000040)={0xaa, 0x20})<br />
r6 = userfaultfd(0x80801)<br />
ioctl$UFFDIO_API(r6, 0xc018aa3f, &(0x7f0000000140))<br />
ioctl$UFFDIO_REGISTER(r6, 0xc020aa00, &(0x7f0000000100)={{&(0x7f0000ffc000/0x4000)=nil, 0x4000}, 0x2})<br />
ioctl$UFFDIO_ZEROPAGE(r5, 0xc020aa04, &(0x7f0000000000)={{&(0x7f0000ffd000/0x1000)=nil, 0x1000}})<br />
r7 = bpf$PROG_LOAD(0x5, &(0x7f0000000140)={0x2, 0x3, &(0x7f0000000200)=ANY=[@ANYBLOB="1800000000120000000000000000000095"], &(0x7f0000000000)=&#39;GPL\x00&#39;, 0x7, 0x0, 0x0, 0x0, 0x0, &#39;\x00&#39;, 0x0, @fallback=0x30, 0xffffffffffffffff, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x10, 0x0, @void, @value}, 0x94)<br />
bpf$BPF_LINK_CREATE_XDP(0x1c, &(0x7f0000000040)={r7, 0x0, 0x30, 0x1e, @val=@uprobe_multi={&(0x7f0000000080)=&#39;./file0\x00&#39;, &(0x7f0000000100)=[0x2], 0x0, 0x0, 0x1}}, 0x40)<br />
<br />
The cause is that zero pfn is set to the PTE without increasing the RSS<br />
count in mfill_atomic_pte_zeropage() and the refcount of zero folio does<br />
not increase accordingly. Then, the operation on the same pfn is performed<br />
in uprobe_write_opcode()->__replace_page() to unconditional decrease the<br />
RSS count and old_folio&#39;s refcount.<br />
<br />
Therefore, two bugs are introduced:<br />
<br />
1. The RSS count is incorrect, when process exit, the check_mm() report<br />
error "Bad rss-count".<br />
<br />
2. The reserved folio (zero folio) is freed when folio->refcount is zero,<br />
then free_pages_prepare->free_page_is_bad() report error<br />
"Bad page state".<br />
<br />
There is more, the following warning could also theoretically be triggered:<br />
<br />
__replace_page()<br />
-> ...<br />
-> folio_remove_rmap_pte()<br />
-> VM_WARN_ON_FOLIO(is_zero_folio(folio), folio)<br />
<br />
Considering that uprobe hit on the zero folio is a very rare case, just<br />
reject zero old folio immediately after get_user_page_vma_remote().<br />
<br />
[ mingo: Cleaned up the changelog ]
Severity CVSS v4.0: Pending analysis
Last modification:
03/11/2025