Re: [PATCH -next v2] uprobes: fix two zero old_folio bugs in __replace_page()

[David Hildenbrand <david@redhat.com>]

On 21.02.25 02:50, Tong Tiangen wrote:
> We triggered the following error logs in syzkaller test:
> 
>    BUG: Bad page state in process syz.7.38  pfn:1eff3
>    page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x1eff3
>    flags: 0x3fffff00004004(referenced|reserved|node=0|zone=1|lastcpupid=0x1fffff)
>    raw: 003fffff00004004 ffffe6c6c07bfcc8 ffffe6c6c07bfcc8 0000000000000000
>    raw: 0000000000000000 0000000000000000 00000000fffffffe 0000000000000000
>    page dumped because: PAGE_FLAGS_CHECK_AT_FREE flag(s) set
>    Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1ubuntu1.1 04/01/2014
>    Call Trace:
>     <TASK>
>     dump_stack_lvl+0x32/0x50
>     bad_page+0x69/0xf0
>     free_unref_page_prepare+0x401/0x500
>     free_unref_page+0x6d/0x1b0
>     uprobe_write_opcode+0x460/0x8e0
>     install_breakpoint.part.0+0x51/0x80
>     register_for_each_vma+0x1d9/0x2b0
>     __uprobe_register+0x245/0x300
>     bpf_uprobe_multi_link_attach+0x29b/0x4f0
>     link_create+0x1e2/0x280
>     __sys_bpf+0x75f/0xac0
>     __x64_sys_bpf+0x1a/0x30
>     do_syscall_64+0x56/0x100
>     entry_SYSCALL_64_after_hwframe+0x78/0xe2
> 
>     BUG: Bad rss-counter state mm:00000000452453e0 type:MM_FILEPAGES val:-1
> 
> The following syzkaller test case can be used to reproduce:
> 
>    r2 = creat(&(0x7f0000000000)='./file0\x00', 0x8)
>    write$nbd(r2, &(0x7f0000000580)=ANY=[], 0x10)
>    r4 = openat(0xffffffffffffff9c, &(0x7f0000000040)='./file0\x00', 0x42, 0x0)
>    mmap$IORING_OFF_SQ_RING(&(0x7f0000ffd000/0x3000)=nil, 0x3000, 0x0, 0x12, r4, 0x0)
>    r5 = userfaultfd(0x80801)
>    ioctl$UFFDIO_API(r5, 0xc018aa3f, &(0x7f0000000040)={0xaa, 0x20})
>    r6 = userfaultfd(0x80801)
>    ioctl$UFFDIO_API(r6, 0xc018aa3f, &(0x7f0000000140))
>    ioctl$UFFDIO_REGISTER(r6, 0xc020aa00, &(0x7f0000000100)={{&(0x7f0000ffc000/0x4000)=nil, 0x4000}, 0x2})
>    ioctl$UFFDIO_ZEROPAGE(r5, 0xc020aa04, &(0x7f0000000000)={{&(0x7f0000ffd000/0x1000)=nil, 0x1000}})
>    r7 = bpf$PROG_LOAD(0x5, &(0x7f0000000140)={0x2, 0x3, &(0x7f0000000200)=ANY=[@ANYBLOB="1800000000120000000000000000000095"], &(0x7f0000000000)='GPL\x00', 0x7, 0x0, 0x0, 0x0, 0x0, '\x00', 0x0, @fallback=0x30, 0xffffffffffffffff, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x10, 0x0, @void, @value}, 0x94)
>    bpf$BPF_LINK_CREATE_XDP(0x1c, &(0x7f0000000040)={r7, 0x0, 0x30, 0x1e, @val=@uprobe_multi={&(0x7f0000000080)='./file0\x00', &(0x7f0000000100)=[0x2], 0x0, 0x0, 0x1}}, 0x40)
> 
> The cause is that zero pfn is set to the pte without increasing the rss
> count in mfill_atomic_pte_zeropage() and the refcount of zero folio does
> not increase accordingly. Then, the operation on the same pfn is performed
> in uprobe_write_opcode()->__replace_page() to unconditional decrease the
> rss count and old_folio's refcount.
> 
> Therefore, two bugs are introduced:
> 1. The rss count is incorrect, when process exit, the check_mm() report
>     error "Bad rss-count".
> 2. The reserved folio (zero folio) is freed when folio->refcount is zero,
>     then free_pages_prepare->free_page_is_bad() report error
>     "Bad page state".

Well, there is more, like triggering the

	VM_WARN_ON_FOLIO(is_zero_folio(folio), folio);

in __folio_rmap_sanity_checks() I assume.

So maybe just call the patch

	"uprobes: reject the share zeropage in uprobe_write_opcode)()"

Thanks!

-- 
Cheers,

David / dhildenb