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

[Ariel Miculas <ariel.miculas@belden.com>]

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@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
+
 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
 
@@ -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
+	/*
+	 * 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)];
+#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;
-- 
2.36.1