[PATCH v1 1/2] RISC-V: skip parsing multi-letter extensions starting with caps

[Conor Dooley]

Yangyu Chen reported that if an multi-letter extension begins with a
capital letter the parser will treat the remainder of that multi-letter
extension as single-letter extensions.

Certain versions of rocket-chip will export devicetree containing
rv64ima_Zifencei, which is parsed by the kernel as rv64imafc.

While capital letters in riscv,isa are invalid and the validation of
devicetree's isn't the kernel's job, we should behave more gracefully
here. Rather than abort parsing on meeting a capital letter, mark the
extension as an error & allow the parser to skip ahead to the next
extension.

Reported-by: Yangyu Chen <cyy at cyyself.name>
Link: https://lore.kernel.org/all/tencent_1647475C9618C390BEC601BE2CC1206D0C07@qq.com/
Fixes: 2a31c54be097 ("RISC-V: Minimal parser for "riscv, isa" strings")
Signed-off-by: Conor Dooley <conor.dooley at microchip.com>
---
 arch/riscv/kernel/cpufeature.c | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
index 52585e088873..93850540b0b4 100644
--- a/arch/riscv/kernel/cpufeature.c
+++ b/arch/riscv/kernel/cpufeature.c
@@ -142,6 +142,10 @@ void __init riscv_fill_hwcap(void)
 			const char *ext_end = isa;
 			bool ext_long = false, ext_err = false;
 
+			if (unlikely(!islower(*ext))) {
+				ext_err = true;
+			}
+
 			switch (*ext) {
 			case 's':
 				/**
@@ -156,6 +160,15 @@ void __init riscv_fill_hwcap(void)
 					break;
 				}
 				fallthrough;
+			case 'S':
+			case 'X':
+			case 'Z':
+				/*
+				 * As the riscv,isa string must be lower-case,
+				 * S, X and Z are not valid characters. Parse
+				 * the invalid extension anyway, to skip ahead
+				 * to the next valid one.
+				 */
 			case 'x':
 			case 'z':
 				ext_long = true;
@@ -185,10 +198,8 @@ void __init riscv_fill_hwcap(void)
 				++ext_end;
 				break;
 			default:
-				if (unlikely(!islower(*ext))) {
-					ext_err = true;
+				if (unlikely(ext_err))
 					break;
-				}
 				/* Find next extension */
 				if (!isdigit(*isa))
 					break;
-- 
2.39.2