[PATCH v4 01/20] arm64: fpsimd: Fix type mismatch in sve_{save,load}_state()

[Mark Rutland]

The sve_save_state() and sve_load_state() functions take a 32-bit int
argument that describes whether to save/restore the FFR. Their assembly
implementations consume the entire 64-bit register containing this
32-bit value, and will attempt to save/restore the FFR if any bit of
that 64-bit register is non-zero.

Per the AAPCS64 parameter passing rules, the callee is responsible for
any necessary widening, and the upper 32-bits are permitted to contain
arbitrary values. If the upper 32 bits are non-zero, this could result
in an unexpected attempt to save/restore the FFR, and consequently could
lead to unexpected traps/undefs/faults.

In practice compilers are very unlikely to generate code where the upper
32-bits would be non-zero, but they are permitted to do so.

Fix this by only consuming the low 32 bits of the register, and update
comments accordingly.

The hyp code __sve_save_state() and __sve_restore_state() functions
don't have the same latent bug as they override the full 64-bit register
containing the argument.

Fixes: 9f5848665788 ("arm64/sve: Make access to FFR optional")
Signed-off-by: Mark Rutland <mark.rutland at arm.com>
Cc: Catalin Marinas <catalin.marinas at arm.com>
Cc: Fuad Tabba <tabba at google.com>
Cc: James Morse <james.morse at arm.com>
Cc: Marc Zyngier <maz at kernel.org>
Cc: Mark Brown <broonie at kernel.org>
Cc: Oliver Upton <oupton at kernel.org>
Cc: Vladimir Murzin <vladimir.murzin at arm.com>
Cc: Will Deacon <will at kernel.org>
Cc: stable at vger.kernel.org
---
 arch/arm64/kernel/entry-fpsimd.S | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/kernel/entry-fpsimd.S b/arch/arm64/kernel/entry-fpsimd.S
index 6325db1a2179c..cb08d879cb4de 100644
--- a/arch/arm64/kernel/entry-fpsimd.S
+++ b/arch/arm64/kernel/entry-fpsimd.S
@@ -38,10 +38,10 @@ SYM_FUNC_END(fpsimd_load_state)
  *
  * x0 - pointer to buffer for state
  * x1 - pointer to storage for FPSR
- * x2 - Save FFR if non-zero
+ * w2 - Save FFR if non-zero
  */
 SYM_FUNC_START(sve_save_state)
-	sve_save 0, x1, x2, 3
+	sve_save 0, x1, w2, 3
 	ret
 SYM_FUNC_END(sve_save_state)
 
@@ -50,10 +50,10 @@ SYM_FUNC_END(sve_save_state)
  *
  * x0 - pointer to buffer for state
  * x1 - pointer to storage for FPSR
- * x2 - Restore FFR if non-zero
+ * w2 - Restore FFR if non-zero
  */
 SYM_FUNC_START(sve_load_state)
-	sve_load 0, x1, x2, 4
+	sve_load 0, x1, w2, 4
 	ret
 SYM_FUNC_END(sve_load_state)
 
-- 
2.30.2