linux-kernel - [PATCH v2 2/6] MIPS: Guard against any partial write attempt with PTRACE

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Message-ID: <alpine.DEB.2.00.1712111837230.4584@tp.orcam.me.uk>
Date:   Mon, 11 Dec 2017 22:52:15 +0000
From:   "Maciej W. Rozycki" <macro@...s.com>
To:     Ralf Baechle <ralf@...ux-mips.org>,
        James Hogan <james.hogan@...s.com>
CC:     Paul Burton <Paul.Burton@...s.com>,
        Alex Smith <alex@...x-smith.me.uk>,
        Dave Martin <Dave.Martin@....com>, <linux-mips@...ux-mips.org>,
        <linux-kernel@...r.kernel.org>, <stable@...r.kernel.org>
Subject: [PATCH v2 2/6] MIPS: Guard against any partial write attempt with
 PTRACE_SETREGSET

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.

Cc: stable@...r.kernel.org # v3.15+
Fixes: 72b22bbad1e7 ("MIPS: Don't assume 64-bit FP registers for FP regset")
Signed-off-by: Maciej W. Rozycki <macro@...s.com>
---

New in v2.

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

linux-mips-nt-prfpreg-size-bug.diff
Index: linux-sfr-test/arch/mips/kernel/ptrace.c
===================================================================
--- linux-sfr-test.orig/arch/mips/kernel/ptrace.c	2017-12-08 16:21:09.783307000 +0000
+++ linux-sfr-test/arch/mips/kernel/ptrace.c	2017-12-08 16:21:19.498382000 +0000
@@ -507,7 +507,15 @@ 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,
@@ -515,6 +523,8 @@ static int fpr_set(struct task_struct *t
 {
 	int err;
 
+	BUG_ON(count % sizeof(elf_fpreg_t));
+
 	/* XXX fcr31  */
 
 	init_fp_ctx(target);