Re: [PATCH] arm64: module: PLT allowed even !RANDOM_BASE

From: Mark Rutland <mark.rutland@arm.com>
To: Arnd Bergmann <arnd@arndb.de>
Cc: Maria Yu <quic_aiquny@quicinc.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, kernel@quicinc.com,
	linux-arm-msm@vger.kernel.org, Ard Biesheuvel <ardb@kernel.org>
Subject: Re: [PATCH] arm64: module: PLT allowed even !RANDOM_BASE
Date: Mon, 23 Oct 2023 10:02:37 +0100	[thread overview]
Message-ID: <ZTY2rdkY5FfTBUVL@FVFF77S0Q05N> (raw)
In-Reply-To: <56c2d30b-2f25-4613-aab1-00fccbd2fa05@app.fastmail.com>

On Mon, Oct 23, 2023 at 10:08:33AM +0200, Arnd Bergmann wrote:
> On Mon, Oct 23, 2023, at 09:57, Maria Yu wrote:
> > Module PLT feature can be enabled even when RANDOM_BASE is disabled.
> > Break BLT entry counts of relocation types will make module plt entry
> > allocation fail and finally exec format error for even correct and plt
> > allocation available modules.

Has an actual problem been seen in practice, or was this found by looking at
the code?

> >
> > Signed-off-by: Maria Yu <quic_aiquny@quicinc.com>
> 
> Adding Ard Biesheuvel to Cc, as he added the check in commit
> a257e02579e42 ("arm64/kernel: don't ban ADRP to work around
> Cortex-A53 erratum #843419")

I think that the actual mistake is in commit:

  3e35d303ab7d22c4 ("arm64: module: rework module VA range selection")

Prior to that commit, when CONFIG_RANDOMIZE_BASE=n all modules and code had to
be within 128M of each other, and so there were no PLTs necessary for B/BL.
After that commit we can have a 2G module range regardless of
CONFIG_RANDOMIZE_BASE, and PLTs may be necessary for B/BL.

We should have removed the check for !CONFIG_RANDOMIZE_BASE as part of that.

> >  arch/arm64/kernel/module-plts.c | 3 ---
> >  1 file changed, 3 deletions(-)
> >
> > diff --git a/arch/arm64/kernel/module-plts.c 
> > b/arch/arm64/kernel/module-plts.c
> > index bd69a4e7cd60..21a67d52d7a0 100644
> > --- a/arch/arm64/kernel/module-plts.c
> > +++ b/arch/arm64/kernel/module-plts.c
> > @@ -167,9 +167,6 @@ static unsigned int count_plts(Elf64_Sym *syms, 
> > Elf64_Rela *rela, int num,
> >  		switch (ELF64_R_TYPE(rela[i].r_info)) {
> >  		case R_AARCH64_JUMP26:
> >  		case R_AARCH64_CALL26:
> > -			if (!IS_ENABLED(CONFIG_RANDOMIZE_BASE))
> > -				break;
> > -
> >  			/*
> >  			 * We only have to consider branch targets that resolve
> >  			 * to symbols that are defined in a different section.
> 
> I see there are two such checks (in partition_branch_plt_relas()
> and in count_plts()), can you explain in more detail how you
> concluded that one of them is correct but the other one is not?

I believe that the one in partition_branch_plt_relas() needs to go too; that's
just a minor optimization for the case where there shouldn't be any PLTs for
B/BL, and it no longer holds after the module VA range rework.

That was introduced in commit:

  d4e0340919fb9190 ("arm64/module: Optimize module load time by optimizing PLT counting")

Thanks,
Mark.