arm: Add command-line option for enabling CVE-2021-35465 mitigation [PR102035]

Add a new option, -mfix-cmse-cve-2021-35465 and document it.  Enable it
automatically for cortex-m33, cortex-m35p and cortex-m55.

gcc:
	PR target/102035
	* config/arm/arm.opt (mfix-cmse-cve-2021-35465): New option.
	* doc/invoke.texi (Arm Options): Document it.
	* config/arm/arm-cpus.in (quirk_vlldm): New feature bit.
	(ALL_QUIRKS): Add quirk_vlldm.
	(cortex-m33): Add quirk_vlldm.
	(cortex-m35p, cortex-m55): Likewise.
	* config/arm/arm.c (arm_option_override): Enable fix_vlldm if
	targetting an affected CPU and not explicitly controlled on
	the command line.

gcc/config/arm/arm-cpus.in

@@ -186,6 +186,9 @@ define feature quirk_armv6kz
 # Cortex-M3 LDRD quirk.
 define feature quirk_cm3_ldrd
 
+# v8-m/v8.1-m VLLDM errata.
+define feature quirk_vlldm
+
 # Don't use .cpu assembly directive
 define feature quirk_no_asmcpu
 
@@ -322,7 +325,7 @@ define implied vfp_base MVE MVE_FP ALL_FP
 # architectures.
 # xscale isn't really a 'quirk', but it isn't an architecture either and we
 # need to ignore it for matching purposes.
-define fgroup ALL_QUIRKS   quirk_no_volatile_ce quirk_armv6kz quirk_cm3_ldrd xscale quirk_no_asmcpu
+define fgroup ALL_QUIRKS   quirk_no_volatile_ce quirk_armv6kz quirk_cm3_ldrd quirk_vlldm xscale quirk_no_asmcpu
 
 define fgroup IGNORE_FOR_MULTILIB cdecp0 cdecp1 cdecp2 cdecp3 cdecp4 cdecp5 cdecp6 cdecp7
 
@@ -1571,6 +1574,7 @@ begin cpu cortex-m33
  architecture armv8-m.main+dsp+fp
  option nofp remove ALL_FP
  option nodsp remove armv7em
+ isa quirk_vlldm
  costs v7m
 end cpu cortex-m33
 
@@ -1580,6 +1584,7 @@ begin cpu cortex-m35p
  architecture armv8-m.main+dsp+fp
  option nofp remove ALL_FP
  option nodsp remove armv7em
+ isa quirk_vlldm
  costs v7m
 end cpu cortex-m35p
 
@@ -1591,7 +1596,7 @@ begin cpu cortex-m55
  option nomve remove mve mve_float
  option nofp remove ALL_FP mve_float
  option nodsp remove MVE mve_float
- isa quirk_no_asmcpu
+ isa quirk_no_asmcpu quirk_vlldm
  costs v7m
  vendor 41
 end cpu cortex-m55

gcc/config/arm/arm.c

@@ -3616,6 +3616,15 @@ arm_option_override (void)
 	fix_cm3_ldrd = 0;
     }
 
+  /* Enable fix_vlldm by default if required.  */
+  if (fix_vlldm == 2)
+    {
+      if (bitmap_bit_p (arm_active_target.isa, isa_bit_quirk_vlldm))
+	fix_vlldm = 1;
+      else
+	fix_vlldm = 0;
+    }
+
   /* Hot/Cold partitioning is not currently supported, since we can't
      handle literal pool placement in that case.  */
   if (flag_reorder_blocks_and_partition)

gcc/config/arm/arm.opt

@@ -268,6 +268,10 @@ Target Var(fix_cm3_ldrd) Init(2)
 Avoid overlapping destination and address registers on LDRD instructions
 that may trigger Cortex-M3 errata.
 
+mfix-cmse-cve-2021-35465
+Target Var(fix_vlldm) Init(2)
+Mitigate issues with VLLDM on some M-profile devices (CVE-2021-35465).
+
 munaligned-access
 Target Var(unaligned_access) Init(2) Save
 Enable unaligned word and halfword accesses to packed data.

gcc/doc/invoke.texi

@@ -808,6 +808,7 @@ Objective-C and Objective-C++ Dialects}.
 -mverbose-cost-dump @gol
 -mpure-code @gol
 -mcmse @gol
+-mfix-cmse-cve-2021-35465 @gol
 -mfdpic}
 
 @emph{AVR Options}
@@ -20743,6 +20744,14 @@ Generate secure code as per the "ARMv8-M Security Extensions: Requirements on
 Development Tools Engineering Specification", which can be found on
 @url{https://developer.arm.com/documentation/ecm0359818/latest/}.
 
+@item -mfix-cmse-cve-2021-35465
+@opindex mfix-cmse-cve-2021-35465
+Mitigate against a potential security issue with the @code{VLLDM} instruction
+in some M-profile devices when using CMSE (CVE-2021-365465).  This option is
+enabled by default when the option @option{-mcpu=} is used with
+@code{cortex-m33}, @code{cortex-m35p} or @code{cortex-m55}.  The option
+@option{-mno-fix-cmse-cve-2021-35465} can be used to disable the mitigation.
+
 @item -mfdpic
 @itemx -mno-fdpic
 @opindex mfdpic