[PATCH v3 18/60] arm64: idreg-override: Prepare for place relative reloc patching

Ard Biesheuvel
2023-03-07T14:08:52Z

The ID reg override handling code uses a rather elaborate data structure
that relies on statically initialized absolute address values in pointer
fields. This means that this code cannot run until relocation fixups
have been applied, and this is unfortunate, because it means we cannot
discover overrides for KASLR or LVA/LPA without creating the kernel
mapping and performing the relocations first.

This can be solved by switching to place-relative relocations, which can
be applied by the linker at build time. This means some additional
arithmetic is required when dereferencing these pointers, as we can no
longer dereference the pointer members directly.

So let's implement this for idreg-override.c in a preliminary way, i.e.,
convert all the references in code to use a special accessor that
produces the correct absolute value at runtime.

To preserve the strong type checking for the static initializers, use
union types for representing the hybrid quantities.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
arch/arm64/kernel/idreg-override.c | 98 +++++++++++++-------
1 file changed, 65 insertions(+), 33 deletions(-)

diff --git a/arch/arm64/kernel/idreg-override.c b/arch/arm64/kernel/idreg-override.c
index b6e90ee6857eb758..fc9ed722621412bf 100644
--- a/arch/arm64/kernel/idreg-override.c
+++ b/arch/arm64/kernel/idreg-override.c
@@ -21,14 +21,32 @@

static u64 __boot_status __initdata;

+// temporary __prel64 related definitions
+// to be removed when this code is moved under pi/
+
+#define __prel64_initconst __initconst
+
+typedef void *prel64_t;
+
+static void *prel64_to_pointer(const prel64_t *p)
+{
+ return *p;
+}
+
struct ftr_set_desc {
char name[FTR_DESC_NAME_LEN];
- struct arm64_ftr_override *override;
+ union {
+ struct arm64_ftr_override *override;
+ prel64_t override_prel;
+ };
struct {
char name[FTR_DESC_FIELD_LEN];
u8 shift;
u8 width;
- bool (*filter)(u64 val);
+ union {
+ bool (*filter)(u64 val);
+ prel64_t filter_prel;
+ };
} fields[];
};

@@ -46,7 +64,7 @@ static bool __init mmfr1_vh_filter(u64 val)
val == 0);
}

-static const struct ftr_set_desc mmfr1 __initconst = {
+static const struct ftr_set_desc mmfr1 __prel64_initconst = {
.name = "id_aa64mmfr1",
.override = &id_aa64mmfr1_override,
.fields = {
@@ -70,7 +88,7 @@ static bool __init pfr0_sve_filter(u64 val)
return true;
}

-static const struct ftr_set_desc pfr0 __initconst = {
+static const struct ftr_set_desc pfr0 __prel64_initconst = {
.name = "id_aa64pfr0",
.override = &id_aa64pfr0_override,
.fields = {
@@ -94,7 +112,7 @@ static bool __init pfr1_sme_filter(u64 val)
return true;
}

-static const struct ftr_set_desc pfr1 __initconst = {
+static const struct ftr_set_desc pfr1 __prel64_initconst = {
.name = "id_aa64pfr1",
.override = &id_aa64pfr1_override,
.fields = {
@@ -105,7 +123,7 @@ static const struct ftr_set_desc pfr1 __initconst = {
},
};

-static const struct ftr_set_desc isar1 __initconst = {
+static const struct ftr_set_desc isar1 __prel64_initconst = {
.name = "id_aa64isar1",
.override = &id_aa64isar1_override,
.fields = {
@@ -117,7 +135,7 @@ static const struct ftr_set_desc isar1 __initconst = {
},
};

-static const struct ftr_set_desc isar2 __initconst = {
+static const struct ftr_set_desc isar2 __prel64_initconst = {
.name = "id_aa64isar2",
.override = &id_aa64isar2_override,
.fields = {
@@ -127,7 +145,7 @@ static const struct ftr_set_desc isar2 __initconst = {
},
};

-static const struct ftr_set_desc smfr0 __initconst = {
+static const struct ftr_set_desc smfr0 __prel64_initconst = {
.name = "id_aa64smfr0",
.override = &id_aa64smfr0_override,
.fields = {
@@ -138,7 +156,7 @@ static const struct ftr_set_desc smfr0 __initconst = {
},
};

-static const struct ftr_set_desc sw_features __initconst = {
+static const struct ftr_set_desc sw_features __prel64_initconst = {
.name = "arm64_sw",
.override = &arm64_sw_feature_override,
.fields = {
@@ -147,14 +165,17 @@ static const struct ftr_set_desc sw_features __initconst = {
},
};

-static const struct ftr_set_desc * const regs[] __initconst = {
- &mmfr1,
- &pfr0,
- &pfr1,
- &isar1,
- &isar2,
- &smfr0,
- &sw_features,
+static const union {
+ const struct ftr_set_desc *reg;
+ prel64_t reg_prel;
+} regs[] __prel64_initconst = {
+ { .reg = &mmfr1 },
+ { .reg = &pfr0 },
+ { .reg = &pfr1 },
+ { .reg = &isar1 },
+ { .reg = &isar2 },
+ { .reg = &smfr0 },
+ { .reg = &sw_features },
};

static const struct {
@@ -194,15 +215,20 @@ static void __init match_options(const char *cmdline)
int i;

for (i = 0; i < ARRAY_SIZE(regs); i++) {
+ const struct ftr_set_desc *reg = prel64_to_pointer(&regs[i].reg_prel);
+ struct arm64_ftr_override *override;
int f;

- for (f = 0; strlen(regs[i]->fields[f].name); f++) {
- u64 shift = regs[i]->fields[f].shift;
- u64 width = regs[i]->fields[f].width ?: 4;
+ override = prel64_to_pointer(&reg->override_prel);
+
+ for (f = 0; strlen(reg->fields[f].name); f++) {
+ u64 shift = reg->fields[f].shift;
+ u64 width = reg->fields[f].width ?: 4;
u64 mask = GENMASK_ULL(shift + width - 1, shift);
+ bool (*filter)(u64 val);
u64 v;

- if (find_field(cmdline, regs[i], f, &v))
+ if (find_field(cmdline, reg, f, &v))
continue;

/*
@@ -210,16 +236,16 @@ static void __init match_options(const char *cmdline)
* it by setting the value to the all-ones while
* clearing the mask... Yes, this is fragile.
*/
- if (regs[i]->fields[f].filter &&
- !regs[i]->fields[f].filter(v)) {
- regs[i]->override->val |= mask;
- regs[i]->override->mask &= ~mask;
+ filter = prel64_to_pointer(&reg->fields[f].filter_prel);
+ if (filter && !filter(v)) {
+ override->val |= mask;
+ override->mask &= ~mask;
continue;
}

- regs[i]->override->val &= ~mask;
- regs[i]->override->val |= (v << shift) & mask;
- regs[i]->override->mask |= mask;
+ override->val &= ~mask;
+ override->val |= (v << shift) & mask;
+ override->mask |= mask;

return;
}
@@ -293,11 +319,16 @@ void init_feature_override(u64 boot_status);

asmlinkage void __init init_feature_override(u64 boot_status)
{
+ struct arm64_ftr_override *override;
+ const struct ftr_set_desc *reg;
int i;

for (i = 0; i < ARRAY_SIZE(regs); i++) {
- regs[i]->override->val = 0;
- regs[i]->override->mask = 0;
+ reg = prel64_to_pointer(&regs[i].reg_prel);
+ override = prel64_to_pointer(&reg->override_prel);
+
+ override->val = 0;
+ override->mask = 0;
}

__boot_status = boot_status;
@@ -305,8 +336,9 @@ asmlinkage void __init init_feature_override(u64 boot_status)
parse_cmdline();

for (i = 0; i < ARRAY_SIZE(regs); i++) {
- dcache_clean_inval_poc((unsigned long)regs[i]->override,
- (unsigned long)regs[i]->override +
- sizeof(*regs[i]->override));
+ reg = prel64_to_pointer(&regs[i].reg_prel);
+ override = prel64_to_pointer(&reg->override_prel);
+ dcache_clean_inval_poc((unsigned long)override,
+ (unsigned long)(override + 1));
}
}
--
2.39.2