Re: [PATCH] riscv: Avoid fortify warning in syscall_get_arguments()

[Nathan Chancellor <nathan@kernel.org>]

On Tue, Apr 15, 2025 at 07:54:04AM +0200, Alexandre Ghiti wrote:
> Hi Nathan,
> 
> On 09/04/2025 23:24, Nathan Chancellor wrote:
> > When building with CONFIG_FORTIFY_SOURCE=y and W=1, there is a warning
> > because of the memcpy() in syscall_get_arguments():
> > 
> >    In file included from include/linux/string.h:392,
> >                     from include/linux/bitmap.h:13,
> >                     from include/linux/cpumask.h:12,
> >                     from arch/riscv/include/asm/processor.h:55,
> >                     from include/linux/sched.h:13,
> >                     from kernel/ptrace.c:13:
> >    In function 'fortify_memcpy_chk',
> >        inlined from 'syscall_get_arguments.isra' at arch/riscv/include/asm/syscall.h:66:2:
> >    include/linux/fortify-string.h:580:25: error: call to '__read_overflow2_field' declared with attribute warning: detected read beyond size of field (2nd parameter); maybe use struct_group()? [-Werror=attribute-warning]
> >      580 |                         __read_overflow2_field(q_size_field, size);
> >          |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >    cc1: all warnings being treated as errors
> > 
> > The fortified memcpy() routine enforces that the source is not overread
> > and the destination is not overwritten if the size of either field and
> > the size of the copy are known at compile time. The memcpy() in
> > syscall_get_arguments() intentionally overreads from a1 to a5 in
> > 'struct pt_regs' but this is bigger than the size of a1.
> > 
> > Normally, this could be solved by wrapping a1 through a5 with
> > struct_group() but there was already a struct_group() applied to these
> > members in commit bba547810c66 ("riscv: tracing: Fix
> > __write_overflow_field in ftrace_partial_regs()").
> > 
> > Just avoid memcpy() altogether and write the copying of args from regs
> > manually, which clears up the warning at the expense of three extra
> > lines of code.
> > 
> > Signed-off-by: Nathan Chancellor <nathan@kernel.org>
> > ---
> > I omitted a Fixes tag because I think this has always been an overread
> > if I understand correctly but it is only the addition of the checks from
> > commit f68f2ff91512 ("fortify: Detect struct member overflows in
> > memcpy() at compile-time") that it becomes a noticeable issue.
> > 
> > This came out of a discussion from the addition of
> > syscall_set_arguments(), where the same logic causes a more noticeable
> > fortify warning because it happens without W=1, as it is an overwrite:
> > https://lore.kernel.org/20250408213131.GA2872426@ax162/
> > ---
> >   arch/riscv/include/asm/syscall.h | 7 +++++--
> >   1 file changed, 5 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/riscv/include/asm/syscall.h b/arch/riscv/include/asm/syscall.h
> > index 121fff429dce66b31fe79b691b8edd816c8019e9..eceabf59ae482aa1832b09371ddb3ba8cd65f91d 100644
> > --- a/arch/riscv/include/asm/syscall.h
> > +++ b/arch/riscv/include/asm/syscall.h
> > @@ -62,8 +62,11 @@ static inline void syscall_get_arguments(struct task_struct *task,
> >   					 unsigned long *args)
> >   {
> >   	args[0] = regs->orig_a0;
> > -	args++;
> > -	memcpy(args, &regs->a1, 5 * sizeof(args[0]));
> > +	args[1] = regs->a1;
> > +	args[2] = regs->a2;
> > +	args[3] = regs->a3;
> > +	args[4] = regs->a4;
> > +	args[5] = regs->a5;
> >   }
> >   static inline int syscall_get_arch(struct task_struct *task)
> > 
> > ---
> > base-commit: 0af2f6be1b4281385b618cb86ad946eded089ac8
> > change-id: 20250409-riscv-avoid-fortify-warning-syscall_get_arguments-19c0495d4ed7
> > 
> > Best regards,
> 
> Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
> 
> IIUC, Andrew took this patch, if that changes, please let me know and I'll
> merge it through the riscv tree.

Thanks, I had Andrew drop it so that it could go via the riscv tree so
please pick it up when you can.

https://lore.kernel.org/20250411211833.E3DD1C4CEE2@smtp.kernel.org/

Cheers,
Nathan

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