create_init_idmap() could be called before .bss section initialization which is done in early_map_kernel(). Therefore, data/test_prot could be set incorrectly by PTE_MAYBE_NG macro.
PTE_MAYBE_NG macro set NG bit according to value of "arm64_use_ng_mappings". and this variable places in .bss section.
# llvm-objdump-21 --syms vmlinux-gcc | grep arm64_use_ng_mappings ffff800082f242a8 g O .bss 0000000000000001 arm64_use_ng_mappings
If .bss section doesn't initialized, "arm64_use_ng_mappings" would be set with garbage value and then the text_prot or data_prot could be set incorrectly.
Here is what i saw with kernel compiled via llvm-21
// create_init_idmap() ffff80008255c058: d10103ff sub sp, sp, #0x40 ffff80008255c05c: a9017bfd stp x29, x30, [sp, #0x10] ffff80008255c060: a90257f6 stp x22, x21, [sp, #0x20] ffff80008255c064: a9034ff4 stp x20, x19, [sp, #0x30] ffff80008255c068: 910043fd add x29, sp, #0x10 ffff80008255c06c: 90003fc8 adrp x8, 0xffff800082d54000 ffff80008255c070: d280e06a mov x10, #0x703 // =1795 ffff80008255c074: 91400409 add x9, x0, #0x1, lsl #12 // =0x1000 ffff80008255c078: 394a4108 ldrb w8, [x8, #0x290] ------------- (1) ffff80008255c07c: f2e00d0a movk x10, #0x68, lsl #48 ffff80008255c080: f90007e9 str x9, [sp, #0x8] ffff80008255c084: aa0103f3 mov x19, x1 ffff80008255c088: aa0003f4 mov x20, x0 ffff80008255c08c: 14000000 b 0xffff80008255c08c <__pi_create_init_idmap+0x34> ffff80008255c090: aa082d56 orr x22, x10, x8, lsl #11 -------- (2)
Note, (1) is load the arm64_use_ng_mappings value in w8 and (2) is set the text or data prot with the w8 value to set PTE_NG bit. If .bss section doesn't initialized, x8 can include garbage value -- In case of some platform, x8 loaded with 0xcf -- it could generate wrong mapping. (i.e) text_prot is expected with PAGE_KERNEL_ROX(0x0040000000000F83) but with garbage x8 -- 0xcf, it sets with (0x0040000000067F83) and This makes boot failure with translation fault.
This error cannot happen according to code generated by compiler.
here is the case of gcc: ffff80008260a940 <__pi_create_init_idmap>: ffff80008260a940: d100c3ff sub sp, sp, #0x30 ffff80008260a944: aa0003ed mov x13, x0 ffff80008260a948: 91400400 add x0, x0, #0x1, lsl #12 // =0x1000 ffff80008260a94c: a9017bfd stp x29, x30, [sp, #0x10] ffff80008260a950: 910043fd add x29, sp, #0x10 ffff80008260a954: f90017e0 str x0, [sp, #0x28] ffff80008260a958: d00048c0 adrp x0, 0xffff800082f24000 <reset_devices> ffff80008260a95c: 394aa000 ldrb w0, [x0, #0x2a8] ffff80008260a960: 37000640 tbnz w0, #0x0, 0xffff80008260aa28 <__pi_create_init_idmap+0xe8> ---(3) ffff80008260a964: d280f060 mov x0, #0x783 // =1923 ffff80008260a968: d280e062 mov x2, #0x703 // =1795 ffff80008260a96c: f2e00800 movk x0, #0x40, lsl #48 ffff80008260a970: f2e00d02 movk x2, #0x68, lsl #48 ffff80008260a974: aa2103e4 mvn x4, x1 ffff80008260a978: 8a210049 bic x9, x2, x1 ... ffff80008260aa28: d281f060 mov x0, #0xf83 // =3971 ffff80008260aa2c: d281e062 mov x2, #0xf03 // =3843 ffff80008260aa30: f2e00800 movk x0, #0x40, lsl #48
In case of gcc, according to value of arm64_use_ng_mappings (annoated as(3)), it branches to each prot settup code. However this is also problem since it branches according to garbage value too -- idmapping with incorrect pgprot.
To resolve this, annotate arm64_use_ng_mappings as ro_after_init.
Fixes: 84b04d3e6bdb ("arm64: kernel: Create initial ID map from C code") Cc: stable@vger.kernel.org # 6.9.x Tested-by: Nathan Chancellor nathan@kernel.org Signed-off-by: Yeoreum Yun yeoreum.yun@arm.com --- Since v1: - add comments explaining arm64_use_ng_mappings shouldn't place .bss section - fix type on commit message - https://lore.kernel.org/all/20250502145755.3751405-1-yeoreum.yun@arm.com/
There is another way to solve this problem by setting test/data_prot with _PAGE_DEFAULT which doesn't include PTE_MAYBE_NG with constanst check in create_init_idmap() to be free from arm64_use_ng_mappings. but i think it would be better to change arm64_use_ng_mappings as ro_after_init because it doesn't change after init phase and solve this problem too. --- arch/arm64/kernel/cpufeature.c | 13 ++++++++++++- 1 file changed, 12 insertions(+), 1 deletion(-)
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c index d2104a1e7843..913ae2cead98 100644 --- a/arch/arm64/kernel/cpufeature.c +++ b/arch/arm64/kernel/cpufeature.c @@ -114,7 +114,18 @@ static struct arm64_cpu_capabilities const __ro_after_init *cpucap_ptrs[ARM64_NC
DECLARE_BITMAP(boot_cpucaps, ARM64_NCAPS);
-bool arm64_use_ng_mappings = false; +/* + * The variable arm64_use_ng_mappings should be placed in the .rodata section. + * Otherwise, it would end up in the .bss section, where it is initialized in + * early_map_kernel(). This can cause problems because the PTE_MAYBE_NG macro + * uses this variable, and create_init_idmap() — which might run before + * early_map_kernel() — could end up generating an incorrect idmap table. + * + * In other words, accessing variable placed in .bss section before + * early_map_kernel() will return garbage, + * potentially resulting in a wrong pgprot value. + */ +bool arm64_use_ng_mappings __ro_after_init = false; EXPORT_SYMBOL(arm64_use_ng_mappings);
DEFINE_PER_CPU_READ_MOSTLY(const char *, this_cpu_vector) = vectors; -- LEVI:{C3F47F37-75D8-414A-A8BA-3980EC8A46D7}