Add Morton,Peter and David for discussion//Re: [PATCH -next] uprobes: fix two zero old_folio bugs in __replace_page()

[Tong Tiangen <tongtiangen@huawei.com>]

在 2025/2/17 20:38, Tong Tiangen 写道:
> 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".
> 
> To fix it, add zero folio check before rss counter and refcount decrease.
> 
> Fixes: 7396fa818d62 ("uprobes/core: Make background page replacement logic account for rss_stat counters")
> Fixes: 2b1444983508 ("uprobes, mm, x86: Add the ability to install and remove uprobes breakpoints")
> Signed-off-by: Tong Tiangen <tongtiangen@huawei.com>
> ---
>   kernel/events/uprobes.c | 6 ++++--
>   1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
> index 46ddf3a2334d..ff5694acfa68 100644
> --- a/kernel/events/uprobes.c
> +++ b/kernel/events/uprobes.c
> @@ -213,7 +213,8 @@ static int __replace_page(struct vm_area_struct *vma, unsigned long addr,
>   		dec_mm_counter(mm, MM_ANONPAGES);
>   
>   	if (!folio_test_anon(old_folio)) {
> -		dec_mm_counter(mm, mm_counter_file(old_folio));
> +		if (!is_zero_folio(old_folio))
> +			dec_mm_counter(mm, mm_counter_file(old_folio));
>   		inc_mm_counter(mm, MM_ANONPAGES);
>   	}
>   
> @@ -227,7 +228,8 @@ static int __replace_page(struct vm_area_struct *vma, unsigned long addr,
>   	if (!folio_mapped(old_folio))
>   		folio_free_swap(old_folio);
>   	page_vma_mapped_walk_done(&pvmw);
> -	folio_put(old_folio);
> +	if (!is_zero_folio(old_folio))
> +		folio_put(old_folio);
>   
>   	err = 0;
>    unlock: