On Fri, Mar 20, 2026 at 03:59:42PM +0100, Ard Biesheuvel wrote: > From: Ard Biesheuvel <[email protected]> > > Nothing in the fixmap or in its vicinity requires executable > permissions, and given that it is placed at exactly 1 GiB from the end > of the virtual address space, we can safely set the hierarchical XN > attributes on the level 2 table entries covering the fixmap, without > running the risk of inadvertently taking away the executable permissions > on an adjacent mappings. > > This is a hardening measure that reduces the risk of the fixmap being > abused to create executable mappings in the kernel address space. > > Signed-off-by: Ard Biesheuvel <[email protected]> > --- > arch/arm64/mm/fixmap.c | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/arch/arm64/mm/fixmap.c b/arch/arm64/mm/fixmap.c > index c5c5425791da..c3dd3c868cf5 100644 > --- a/arch/arm64/mm/fixmap.c > +++ b/arch/arm64/mm/fixmap.c > @@ -48,7 +48,8 @@ static void __init early_fixmap_init_pte(pmd_t *pmdp, > unsigned long addr) > if (pmd_none(pmd)) { > ptep = bm_pte[BM_PTE_TABLE_IDX(addr)]; > __pmd_populate(pmdp, __pa_symbol(ptep), > - PMD_TYPE_TABLE | PMD_TABLE_AF); > + PMD_TYPE_TABLE | PMD_TABLE_AF | > + PMD_TABLE_PXN | PMD_TABLE_UXN); > } > }
Sashiko reckons this breaks kpti. I think that's valid but I couldn't reproduce it on qemu (maybe it doesn't implement hierarchical permissions). Then I tried FVP and the whole series panics (unrelated to kpti). With kvm-arm.mode=protected, I think kvm_ksym_ref() is lm_alias() and we have kvm_hyp_init_symbols() trying to flush the bss. I'll drop it for now. Unable to handle kernel paging request at virtual address fff00000748f7000 Mem abort info: ESR = 0x0000000096000147 ** replaying previous printk message ** EC = 0x25: DABT (current EL), IL = 32 bits SET = 0, FnV = 0 EA = 0, S1PTW = 0 FSC = 0x07: level 3 translation fault Data abort info: ISV = 0, ISS = 0x00000147, ISS2 = 0x00000000 CM = 1, WnR = 1, TnD = 0, TagAccess = 0 GCS = 0, Overlay = 0, DirtyBit = 0, Xs = 0 swapper pgtable: 4k pages, 52-bit VAs, pgdp=00000000f40ad000 [fff00000748f7000] pgd=18000008fffff403, p4d=18000008ffffe403, pud=18000008ffffd403, pmd=18000008fffe9403, pte=00e80000f48f7406 Internal error: Oops: 0000000096000147 [#1] SMP Modules linked in: CPU: 1 UID: 0 PID: 1 Comm: swapper/0 Not tainted 7.0.0-rc3-00013-g6bb20b972b8c #2 PREEMPT Hardware name: FVP Base RevC (DT) pstate: 81400005 (Nzcv daif +PAN -UAO -TCO +DIT -SSBS BTYPE=--) pc : dcache_clean_inval_poc+0x24/0x48 lr : kvm_arm_init+0xb48/0x1338 sp : ffff80008005bd20 x29: ffff80008005bd60 x28: ffff95c63230b000 x27: 0000000000000004 x26: 0000000000000001 x25: ffff95c631a160c0 x24: 0000000000000008 x23: 0000000000000000 x22: 0000000000000000 x21: fff00000748f7000 x20: 0000000000002000 x19: 0001101131111112 x18: 00000000ffffffff x17: ffff95c6321fdd88 x16: 0000000031427afd x15: 0000000000000100 x14: 0000000000000000 x13: 0000000000077a9a x12: fff000087f805650 x11: fff000087f805630 x10: ffffc1ffe005db08 x9 : 0000000000000000 x8 : 0000000080000000 x7 : ffff80008005bc50 x6 : 000f63580145a000 x5 : ffff95c6322f8000 x4 : 0000000000000000 x3 : 000000000000003f x2 : 0000000000000040 x1 : fff00000748f9000 x0 : fff00000748f7000 Call trace: dcache_clean_inval_poc+0x24/0x48 (P) do_one_initcall+0x60/0x1d4 kernel_init_freeable+0x24c/0x2d4 kernel_init+0x24/0x140 ret_from_fork+0x10/0x20 Code: 9ac32042 d1000443 8a230000 d503201f (d50b7e20) ---[ end trace 0000000000000000 ]--- -- Catalin
