linux-kernel - [PATCH 4.9 09/96] MIPS: Guard against any partial write attempt with PTRACE

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Message-Id: <20180115123405.170317702@linuxfoundation.org>
Date:   Mon, 15 Jan 2018 13:34:08 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, "Maciej W. Rozycki" <macro@...s.com>,
        James Hogan <james.hogan@...s.com>,
        Paul Burton <Paul.Burton@...s.com>,
        Alex Smith <alex@...x-smith.me.uk>,
        Dave Martin <Dave.Martin@....com>, linux-mips@...ux-mips.org,
        Ralf Baechle <ralf@...ux-mips.org>
Subject: [PATCH 4.9 09/96] MIPS: Guard against any partial write attempt with PTRACE_SETREGSET

4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

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

commit dc24d0edf33c3e15099688b6bbdf7bdc24bf6e91 upstream.

Complement commit d614fd58a283 ("mips/ptrace: Preserve previous
registers for short regset write") and ensure that no partial register
write attempt is made with PTRACE_SETREGSET, as we do not preinitialize
any temporaries used to hold incoming register data and consequently
random data could be written.

It is the responsibility of the caller, such as `ptrace_regset', to
arrange for writes to span whole registers only, so here we only assert
that it has indeed happened.

Signed-off-by: Maciej W. Rozycki <macro@...s.com>
Fixes: 72b22bbad1e7 ("MIPS: Don't assume 64-bit FP registers for FP regset")
Cc: James Hogan <james.hogan@...s.com>
Cc: Paul Burton <Paul.Burton@...s.com>
Cc: Alex Smith <alex@...x-smith.me.uk>
Cc: Dave Martin <Dave.Martin@....com>
Cc: linux-mips@...ux-mips.org
Cc: linux-kernel@...r.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/17926/
Signed-off-by: Ralf Baechle <ralf@...ux-mips.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 arch/mips/kernel/ptrace.c |   12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

--- a/arch/mips/kernel/ptrace.c
+++ b/arch/mips/kernel/ptrace.c
@@ -536,7 +536,15 @@ static int fpr_set_msa(struct task_struc
 	return 0;
 }

-/* Copy the supplied NT_PRFPREG buffer to the floating-point context.  */
+/*
+ * Copy the supplied NT_PRFPREG buffer to the floating-point context.
+ *
+ * We optimize for the case where `count % sizeof(elf_fpreg_t) == 0',
+ * which is supposed to have been guaranteed by the kernel before
+ * calling us, e.g. in `ptrace_regset'.  We enforce that requirement,
+ * so that we can safely avoid preinitializing temporaries for
+ * partial register writes.
+ */
 static int fpr_set(struct task_struct *target,
 		   const struct user_regset *regset,
 		   unsigned int pos, unsigned int count,
@@ -544,6 +552,8 @@ static int fpr_set(struct task_struct *t
 {
 	int err;

+	BUG_ON(count % sizeof(elf_fpreg_t));
+
 	/* XXX fcr31  */

 	init_fp_ctx(target);