linux-kernel - Re: [REGRESSION][BISECTED] erroneous buffer overflow detected in bch2_xattr

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Message-ID: <ZvmxntpAHrAwcKaH@archlinux>
Date: Sun, 29 Sep 2024 21:59:26 +0200
From: Jan Hendrik Farr <kernel@...rr.cc>
To: Kees Cook <kees@...nel.org>
Cc: Bill Wendling <morbo@...gle.com>, Ard Biesheuvel <ardb@...nel.org>,
	Thorsten Blum <thorsten.blum@...lux.com>, kent.overstreet@...ux.dev,
	regressions@...ts.linux.dev, linux-bcachefs@...r.kernel.org,
	linux-hardening@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [REGRESSION][BISECTED] erroneous buffer overflow detected in
 bch2_xattr_validate

On 29 01:33:40, Jan Hendrik Farr wrote:
> On 28 13:50:12, Kees Cook wrote:
> > On Thu, Sep 26, 2024 at 06:30:15PM -0700, Bill Wendling wrote:
> > > On Thu, Sep 26, 2024 at 3:18 PM Bill Wendling <morbo@...gle.com> wrote:
> > > >
> > > > On Thu, Sep 26, 2024 at 12:58 PM Ard Biesheuvel <ardb@...nel.org> wrote:
> > > > >
> > > > > (cc Kees and Bill)
> > > > >
> > > > > On Thu, 26 Sept 2024 at 19:46, Jan Hendrik Farr <kernel@...rr.cc> wrote:
> > > > > >
> > > > > > On 26 19:01:20, Jan Hendrik Farr wrote:
> > > > > > > On 26 18:09:57, Thorsten Blum wrote:
> > > > > > > > On 26. Sep 2024, at 17:28, Thorsten Blum <thorsten.blum@...lux.com> wrote:
> > > > > > > > > On 26. Sep 2024, at 17:14, Jan Hendrik Farr <kernel@...rr.cc> wrote:
> > > [...]
> > > > > > > > >> [    6.252375] memchr: detected buffer overflow: 12 byte read of buffer size 0
> > > [...]
> > > > > > >       bkey_fsck_err_on(memchr(xattr.v->x_name, '\0', xattr.v->x_name_len),
> > > > > > >                        c, xattr_name_invalid_chars,
> > > > > > >                        "xattr name has invalid characters");
> > > [...]
> > 
> > The thing going wrong is that __bdos(xattr.v->x_name, 0) is returning 0.
> > This looks exactly like the bug I minimized here:
> > https://lore.kernel.org/all/202409170436.C3C6E7F7A@keescook/
> > 
> > Since there wasn't an LLVM open bug yet, I've created:
> > https://github.com/llvm/llvm-project/issues/110385
> > 
> > -Kees
> > 
> I found a fix for the issue. Fixes both the issue in this thread as well
> as your reproducer. First thought they might not actually be the same
> issue, but they indeed are. Haven't tested against the issue Thorsten
> linked.
> 
> Havn't run the clang tests on it yet, but it does successfully compile
> my kernel and fix the issue.
> 
> I'll open a PR and give more explanation tomorrow, it's getting
> pretty late over here in CEST.
> 
> 
> Here's the patch to be applied on top of
> https://github.com/llvm/llvm-project
> 
> --
> diff --git a/clang/lib/CodeGen/CGExpr.cpp b/clang/lib/CodeGen/CGExpr.cpp
> index 9166db4c7412..143dd3fcfcf8 100644
> --- a/clang/lib/CodeGen/CGExpr.cpp
> +++ b/clang/lib/CodeGen/CGExpr.cpp
> @@ -1164,15 +1164,15 @@ llvm::Value *CodeGenFunction::EmitLoadOfCountedByField(
>      Res = EmitDeclRefLValue(DRE).getPointer(*this);
>      Res = Builder.CreateAlignedLoad(ConvertType(DRE->getType()), Res,
>                                      getPointerAlign(), "dre.load");
> -  } else if (const MemberExpr *ME = dyn_cast<MemberExpr>(StructBase)) {
> -    LValue LV = EmitMemberExpr(ME);
> -    Address Addr = LV.getAddress();
> -    Res = Addr.emitRawPointer(*this);
>    } else if (StructBase->getType()->isPointerType()) {
>      LValueBaseInfo BaseInfo;
>      TBAAAccessInfo TBAAInfo;
>      Address Addr = EmitPointerWithAlignment(StructBase, &BaseInfo, &TBAAInfo);
>      Res = Addr.emitRawPointer(*this);
> +  } else if (const MemberExpr *ME = dyn_cast<MemberExpr>(StructBase)) {
> +    LValue LV = EmitMemberExpr(ME);
> +    Address Addr = LV.getAddress();
> +    Res = Addr.emitRawPointer(*this);
>    } else {
>      return nullptr;
>    }


Here's the PR: https://github.com/llvm/llvm-project/pull/110437

I hope the way I added the CHECK tags in the test is good and that they if
they don't need manual cleanup, not familiar with the llvm test system.


Best Regards
Jan