[PATCH 4.4 19/99] MIPS: ptrace: Prevent writes to read-only FCSR bits

Greg Kroah-Hartman
2016-06-05T23:00:51Z

From: Maciej W. Rozycki <macro@imgtec.com>

commit abf378be49f38c4d3e23581d3df3fa9f1b1b11d2 upstream.

Correct the cases missed with commit 9b26616c8d9d ("MIPS: Respect the
ISA level in FCSR handling") and prevent writes to read-only FCSR bits
there.

This in particular applies to FP context initialisation where any IEEE
754-2008 bits preset by `mips_set_personality_nan' are cleared before
the relevant ptrace(2) call takes effect and the PTRACE_POKEUSR request
addressing FPC_CSR where no masking of read-only FCSR bits is done.

Remove the FCSR clearing from FP context initialisation then and unify
PTRACE_POKEUSR/FPC_CSR and PTRACE_SETFPREGS handling, by factoring out
code from `ptrace_setfpregs' and calling it from both places.

This mostly matters to soft float configurations where the emulator can
be switched this way to a mode which should not be accessible and cannot
be set with the CTC1 instruction. With hard float configurations any
effect is transient anyway as read-only bits will retain their values at
the time the FP context is restored.

Signed-off-by: Maciej W. Rozycki <macro@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/13239/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
arch/mips/kernel/ptrace.c | 28 +++++++++++++++++++---------
1 file changed, 19 insertions(+), 9 deletions(-)

--- a/arch/mips/kernel/ptrace.c
+++ b/arch/mips/kernel/ptrace.c
@@ -57,8 +57,7 @@ static void init_fp_ctx(struct task_stru
/* Begin with data registers set to all 1s... */
memset(&target->thread.fpu.fpr, ~0, sizeof(target->thread.fpu.fpr));

- /* ...and FCSR zeroed */
- target->thread.fpu.fcr31 = 0;
+ /* FCSR has been preset by `mips_set_personality_nan'. */

/*
* Record that the target has "used" math, such that the context
@@ -80,6 +79,22 @@ void ptrace_disable(struct task_struct *
}

/*
+ * Poke at FCSR according to its mask. Don't set the cause bits as
+ * this is currently not handled correctly in FP context restoration
+ * and will cause an oops if a corresponding enable bit is set.
+ */
+static void ptrace_setfcr31(struct task_struct *child, u32 value)
+{
+ u32 fcr31;
+ u32 mask;
+
+ value &= ~FPU_CSR_ALL_X;
+ fcr31 = child->thread.fpu.fcr31;
+ mask = boot_cpu_data.fpu_msk31;
+ child->thread.fpu.fcr31 = (value & ~mask) | (fcr31 & mask);
+}
+
+/*
* Read a general register set. We always use the 64-bit format, even
* for 32-bit kernels and for 32-bit processes on a 64-bit kernel.
* Registers are sign extended to fill the available space.
@@ -159,9 +174,7 @@ int ptrace_setfpregs(struct task_struct
{
union fpureg *fregs;
u64 fpr_val;
- u32 fcr31;
u32 value;
- u32 mask;
int i;

if (!access_ok(VERIFY_READ, data, 33 * 8))
@@ -176,10 +189,7 @@ int ptrace_setfpregs(struct task_struct
}

__get_user(value, data + 64);
- value &= ~FPU_CSR_ALL_X;
- fcr31 = child->thread.fpu.fcr31;
- mask = boot_cpu_data.fpu_msk31;
- child->thread.fpu.fcr31 = (value & ~mask) | (fcr31 & mask);
+ ptrace_setfcr31(child, value);

/* FIR may not be written. */

@@ -809,7 +819,7 @@ long arch_ptrace(struct task_struct *chi
break;
#endif
case FPC_CSR:
- child->thread.fpu.fcr31 = data & ~FPU_CSR_ALL_X;
+ ptrace_setfcr31(child, data);
break;
case DSP_BASE ... DSP_BASE + 5: {
dspreg_t *dregs;