From: Ard Biesheuvel ard.biesheuvel@arm.com
commit b2eed9b58811283d00fa861944cb75797d4e52a7 upstream.
The following commit
7290d5809571 ("module: use relative references for __ksymtab entries")
updated the ksymtab handling of some KASLR capable architectures so that ksymtab entries are emitted as pairs of 32-bit relative references. This reduces the size of the entries, but more importantly, it gets rid of statically assigned absolute addresses, which require fixing up at boot time if the kernel is self relocating (which takes a 24 byte RELA entry for each member of the ksymtab struct).
Since ksymtab entries are always part of the same module as the symbol they export, it was assumed at the time that a 32-bit relative reference is always sufficient to capture the offset between a ksymtab entry and its target symbol.
Unfortunately, this is not always true: in the case of per-CPU variables, a per-CPU variable's base address (which usually differs from the actual address of any of its per-CPU copies) is allocated in the vicinity of the ..data.percpu section in the core kernel (i.e., in the per-CPU reserved region which follows the section containing the core kernel's statically allocated per-CPU variables).
Since we randomize the module space over a 4 GB window covering the core kernel (based on the -/+ 4 GB range of an ADRP/ADD pair), we may end up putting the core kernel out of the -/+ 2 GB range of 32-bit relative references of module ksymtab entries that refer to per-CPU variables.
So reduce the module randomization range a bit further. We lose 1 bit of randomization this way, but this is something we can tolerate.
Cc: stable@vger.kernel.org # v4.19+ Signed-off-by: Ard Biesheuvel ard.biesheuvel@arm.com Signed-off-by: Will Deacon will.deacon@arm.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- arch/arm64/kernel/kaslr.c | 6 +++--- arch/arm64/kernel/module.c | 2 +- 2 files changed, 4 insertions(+), 4 deletions(-)
--- a/arch/arm64/kernel/kaslr.c +++ b/arch/arm64/kernel/kaslr.c @@ -145,15 +145,15 @@ u64 __init kaslr_early_init(u64 dt_phys)
if (IS_ENABLED(CONFIG_RANDOMIZE_MODULE_REGION_FULL)) { /* - * Randomize the module region over a 4 GB window covering the + * Randomize the module region over a 2 GB window covering the * kernel. This reduces the risk of modules leaking information * about the address of the kernel itself, but results in * branches between modules and the core kernel that are * resolved via PLTs. (Branches between modules will be * resolved normally.) */ - module_range = SZ_4G - (u64)(_end - _stext); - module_alloc_base = max((u64)_end + offset - SZ_4G, + module_range = SZ_2G - (u64)(_end - _stext); + module_alloc_base = max((u64)_end + offset - SZ_2G, (u64)MODULES_VADDR); } else { /* --- a/arch/arm64/kernel/module.c +++ b/arch/arm64/kernel/module.c @@ -56,7 +56,7 @@ void *module_alloc(unsigned long size) * can simply omit this fallback in that case. */ p = __vmalloc_node_range(size, MODULE_ALIGN, module_alloc_base, - module_alloc_base + SZ_4G, GFP_KERNEL, + module_alloc_base + SZ_2G, GFP_KERNEL, PAGE_KERNEL_EXEC, 0, NUMA_NO_NODE, __builtin_return_address(0));