[PATCH] powerpc/ptrace: Fix buffer overflow when handling PTRACE_PEEKUSER and PTRACE_POKEUSER

[Christophe Leroy]

Le 09/06/2022 à 11:52, Ariel Miculas a écrit :
> This fixes the gdbserver issue on PPC32 described here:
> Link: https://linuxppc-dev.ozlabs.narkive.com/C46DRek4/debug-problems-on-ppc-83xx-target-due-to-changed-struct-task-struct
> 
> On PPC32, the user space code considers the floating point to be an
> array of unsigned int (32 bits) - the index passed in is based on
> this assumption.
> 
> fp_state is a matrix consisting of 32 lines
> /* FP and VSX 0-31 register set /
> struct thread_fp_state {
> u64 fpr[32][TS_FPRWIDTH] attribute((aligned(16)));
> u64 fpscr; / Floating point status */
> };
> 
> On PPC32, PT_FPSCR is defined as: (PT_FPR0 + 2*32 + 1)
> 
> This means the fpr index validation allows a range from 0 to 65, leading
> to out-of-bounds array access. This ends up corrupting
> threads_struct->state, which holds the state of the task. Thus, threads
> incorrectly transition from a running state to a traced state and get
> stuck in that state.
> 
> On PPC32 it's ok to assume that TS_FPRWIDTH is 1 because CONFIG_VSX is
> PPC64 specific. TS_FPROFFSET can be safely ignored, thus the assumption
> that fpr is an array of 32 elements of type u64 holds true.
> 
> Solution taken from arch/powerpc/kernel/ptrace32.c
> 
> Signed-off-by: Ariel Miculas <ariel.miculas at belden.com>
> ---
>   arch/powerpc/kernel/ptrace/ptrace-fpu.c | 31 +++++++++++++++++++++++--
>   1 file changed, 29 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/powerpc/kernel/ptrace/ptrace-fpu.c b/arch/powerpc/kernel/ptrace/ptrace-fpu.c
> index 5dca19361316..93695abbbdfb 100644
> --- a/arch/powerpc/kernel/ptrace/ptrace-fpu.c
> +++ b/arch/powerpc/kernel/ptrace/ptrace-fpu.c
> @@ -6,9 +6,16 @@
>   
>   #include "ptrace-decl.h"
>   
> +#ifdef CONFIG_PPC32
> +/* Macros to workout the correct index for the FPR in the thread struct */
> +#define FPRNUMBER(i) (((i) - PT_FPR0) >> 1)
> +#define FPRHALF(i) (((i) - PT_FPR0) & 1)
> +#define FPRINDEX(i) TS_FPRWIDTH * FPRNUMBER(i) * 2 + FPRHALF(i)
> +#endif

I can't see the benefit of such macros if they are only for PPC32.

> +
>   int ptrace_get_fpr(struct task_struct *child, int index, unsigned long *data)
>   {
> -#ifdef CONFIG_PPC_FPU_REGS
> +#if defined(CONFIG_PPC_FPU_REGS) && !defined(CONFIG_PPC32)
>   	unsigned int fpidx = index - PT_FPR0;
>   #endif

#ifdefs should be avoided as much as possible.

>   
> @@ -17,10 +24,20 @@ int ptrace_get_fpr(struct task_struct *child, int index, unsigned long *data)
>   
>   #ifdef CONFIG_PPC_FPU_REGS
>   	flush_fp_to_thread(child);
> +#ifdef CONFIG_PPC32

Here you could use IS_ENABLED(CONFIG_PPC32), it would also avoid the 
above #ifdef.

> +	/*
> +	 * the user space code considers the floating point
> +	 * to be an array of unsigned int (32 bits) - the
> +	 * index passed in is based on this assumption.
> +	 */
> +	*data = ((unsigned int *)child->thread.fp_state.fpr)
> +		[FPRINDEX(index)];

if I understand FPRINDEX(index) correctly, at the end we have 
FPRINDEX(i) == i, so I can't see the point.

Michael's patch seems easier to understand.

I think if one day we want something common to ppc32 and ppc64, we need 
to use a new macro similar to TS_FPR() but that properly takes ppc32 
into account. Pay attention to not change TS_FPR() as it is used in 
other places where it is valid for both PPC32 and PPC64.


> +#else
>   	if (fpidx < (PT_FPSCR - PT_FPR0))
>   		memcpy(data, &child->thread.TS_FPR(fpidx), sizeof(long));
>   	else
>   		*data = child->thread.fp_state.fpscr;
> +#endif
>   #else
>   	*data = 0;
>   #endif
> @@ -30,7 +47,7 @@ int ptrace_get_fpr(struct task_struct *child, int index, unsigned long *data)
>   
>   int ptrace_put_fpr(struct task_struct *child, int index, unsigned long data)
>   {
> -#ifdef CONFIG_PPC_FPU_REGS
> +#if defined(CONFIG_PPC_FPU_REGS) && !defined(CONFIG_PPC32)
>   	unsigned int fpidx = index - PT_FPR0;
>   #endif
>   
> @@ -39,10 +56,20 @@ int ptrace_put_fpr(struct task_struct *child, int index, unsigned long data)
>   
>   #ifdef CONFIG_PPC_FPU_REGS
>   	flush_fp_to_thread(child);
> +#ifdef CONFIG_PPC32
> +	/*
> +	 * the user space code considers the floating point
> +	 * to be an array of unsigned int (32 bits) - the
> +	 * index passed in is based on this assumption.
> +	 */
> +	((unsigned int *)child->thread.fp_state.fpr)
> +		[FPRINDEX(index)] = data;
> +#else
>   	if (fpidx < (PT_FPSCR - PT_FPR0))
>   		memcpy(&child->thread.TS_FPR(fpidx), &data, sizeof(long));
>   	else
>   		child->thread.fp_state.fpscr = data;
> +#endif
>   #endif
>   
>   	return 0;