linux-kernel - Re: [syzbot] [bpf?] KMSAN: uninit-value in ieee802154_subif_start

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Message-ID: <74c9a88d79b7c65e2fdc2dc1609e13590225cb60.camel@gmail.com>
Date:   Mon, 07 Aug 2023 16:48:40 +0300
From:   Eduard Zingerman <eddyz87@...il.com>
To:     yonghong.song@...ux.dev,
        syzbot <syzbot+d61b595e9205573133b3@...kaller.appspotmail.com>,
        andrii@...nel.org, ast@...nel.org, bpf@...r.kernel.org,
        daniel@...earbox.net, davem@...emloft.net, haoluo@...gle.com,
        hawk@...nel.org, john.fastabend@...il.com, jolsa@...nel.org,
        kpsingh@...nel.org, kuba@...nel.org, linux-kernel@...r.kernel.org,
        martin.lau@...ux.dev, netdev@...r.kernel.org, sdf@...gle.com,
        song@...nel.org, syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [bpf?] KMSAN: uninit-value in
 ieee802154_subif_start_xmit

On Mon, 2023-08-07 at 16:11 +0300, Eduard Zingerman wrote:
[...]
>   $ bpftool prog dump jited id <some-id>
>   bpf_prog_ebeed182d92b487f:
>      0: nopl    (%rax,%rax)
>      5: nop
>      7: pushq   %rbp
>      8: movq    %rsp, %rbp
>      b: subq    $8, %rsp
>     12: movl    $553656332, -8(%rbp)
>     19: movswq  %bp, %rdi            ; <---- Note movswq %bp !
>     1d: addq    $-8, %rdi
>     21: movl    $3, %esi
>     26: cmpq    %rdi, %rsi
>     29: jbe 0x2b
>     2b: callq   0xffffffffe11c484c
>     30: xorl    %eax, %eax
>     32: leave
>     33: retq
> 
> Note jit instruction #19 corresponding to BPF instruction #1, which
> loads truncated and sign-extended value of %rbp's first byte as an
> address of format string.

Correction: sign-extended value of %rbp's first *two* bytes,
disassembly with opcodes:

  19:	movswq	%bp, %rdi
    48 0f bf fd 

[...]