[PATCH] MIPS: prctl: Disallow FRE without FR with PR_SET_FP_MODE requests

[Maciej W. Rozycki]

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

Cc: stable@xxxxxxxxxxxxxxx # 4.0+
Fixes: 9791554b45a2 ("MIPS,prctl: add PR_[GS]ET_FP_MODE prctl options for MIPS")
Signed-off-by: Maciej W. Rozycki <macro@xxxxxxxx>
---
 arch/mips/kernel/process.c |    4 ++++
 1 file changed, 4 insertions(+)

linux-mips-set-process-fp-mode-fr-fre.diff
Index: linux/arch/mips/kernel/process.c
===================================================================
--- linux.orig/arch/mips/kernel/process.c	2018-05-12 22:52:11.000000000 +0100
+++ linux/arch/mips/kernel/process.c	2018-05-12 23:07:15.147112000 +0100
@@ -721,6 +721,10 @@ 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))