linux-kernel - [PATCH 4.4 27/37] MIPS: prctl: Disallow FRE without FR with PR_SET_FP

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Message-Id: <20180605170110.460357621@linuxfoundation.org>
Date:   Tue,  5 Jun 2018 19:01:32 +0200
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>,
        Ralf Baechle <ralf@...ux-mips.org>, linux-mips@...ux-mips.org,
        James Hogan <jhogan@...nel.org>
Subject: [PATCH 4.4 27/37] MIPS: prctl: Disallow FRE without FR with PR_SET_FP_MODE requests

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

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

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

commit 28e4213dd331e944e7fca1954a946829162ed9d4 upstream.

Having PR_FP_MODE_FRE (i.e. Config5.FRE) set without PR_FP_MODE_FR (i.e.
Status.FR) is not supported as the lone purpose of Config5.FRE is to
emulate Status.FR=0 handling on FPU hardware that has Status.FR=1
hardwired[1][2].  Also we do not handle this case elsewhere, and assume
throughout our code that TIF_HYBRID_FPREGS and TIF_32BIT_FPREGS cannot
be set both at once for a task, leading to inconsistent behaviour if
this does happen.

Return unsuccessfully then from prctl(2) PR_SET_FP_MODE calls requesting
PR_FP_MODE_FRE to be set with PR_FP_MODE_FR clear.  This corresponds to
modes allowed by `mips_set_personality_fp'.

References:

[1] "MIPS Architecture For Programmers, Vol. III: MIPS32 / microMIPS32
    Privileged Resource Architecture", Imagination Technologies,
    Document Number: MD00090, Revision 6.02, July 10, 2015, Table 9.69
    "Config5 Register Field Descriptions", p. 262

[2] "MIPS Architecture For Programmers, Volume III: MIPS64 / microMIPS64
    Privileged Resource Architecture", Imagination Technologies,
    Document Number: MD00091, Revision 6.03, December 22, 2015, Table
    9.72 "Config5 Register Field Descriptions", p. 288

Fixes: 9791554b45a2 ("MIPS,prctl: add PR_[GS]ET_FP_MODE prctl options for MIPS")
Signed-off-by: Maciej W. Rozycki <macro@...s.com>
Cc: Ralf Baechle <ralf@...ux-mips.org>
Cc: linux-mips@...ux-mips.org
Cc: <stable@...r.kernel.org> # 4.0+
Patchwork: https://patchwork.linux-mips.org/patch/19327/
Signed-off-by: James Hogan <jhogan@...nel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 arch/mips/kernel/process.c |    4 ++++
 1 file changed, 4 insertions(+)

--- a/arch/mips/kernel/process.c
+++ b/arch/mips/kernel/process.c
@@ -680,6 +680,10 @@ int mips_set_process_fp_mode(struct task
 	if (value & ~known_bits)
 		return -EOPNOTSUPP;

+	/* Setting FRE without FR is not supported.  */
+	if ((value & (PR_FP_MODE_FR | PR_FP_MODE_FRE)) == PR_FP_MODE_FRE)
+		return -EOPNOTSUPP;
+
 	/* Avoid inadvertently triggering emulation */
 	if ((value & PR_FP_MODE_FR) && raw_cpu_has_fpu &&
 	    !(raw_current_cpu_data.fpu_id & MIPS_FPIR_F64))