arm64: fix the doc of RANDOMIZE_MODULE_REGION_FULL

Obviously kaslr is setting the module region to 2GB rather than 4GB since
commit b2eed9b588112 ("arm64/kernel: kaslr: reduce module randomization
range to 2 GB"). So fix the size of region in Kconfig.
On the other hand, even though RANDOMIZE_MODULE_REGION_FULL is not set,
module_alloc() can fall back to a 2GB window if ARM64_MODULE_PLTS is set.
In this case, veneers are still needed. !RANDOMIZE_MODULE_REGION_FULL
doesn't necessarily mean veneers are not needed.
So fix the doc to be more precise to avoid any confusion to the readers
of the code.

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Cc: Qi Liu <liuqi115@huawei.com>
Signed-off-by: Barry Song <song.bao.hua@hisilicon.com>
Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
Link: https://lore.kernel.org/r/20210730125131.13724-1-song.bao.hua@hisilicon.com
Signed-off-by: Will Deacon <will@kernel.org>

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index b5b13a9..fdcd54d 100644 (file)
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -1800,11 +1800,11 @@ config RANDOMIZE_BASE
          If unsure, say N.
 
 config RANDOMIZE_MODULE_REGION_FULL
-       bool "Randomize the module region over a 4 GB range"
+       bool "Randomize the module region over a 2 GB range"
        depends on RANDOMIZE_BASE
        default y
        help
-         Randomizes the location of the module region inside a 4 GB window
+         Randomizes the location of the module region inside a 2 GB window
          covering the core kernel. This way, it is less likely for modules
          to leak information about the location of core kernel data structures
          but it does imply that function calls between modules and the core
@@ -1812,7 +1812,10 @@ config RANDOMIZE_MODULE_REGION_FULL
 
          When this option is not set, the module region will be randomized over
          a limited range that contains the [_stext, _etext] interval of the
-         core kernel, so branch relocations are always in range.
+         core kernel, so branch relocations are almost always in range unless
+         ARM64_MODULE_PLTS is enabled and the region is exhausted. In this
+         particular case of region exhaustion, modules might be able to fall
+         back to a larger 2GB area.
 
 config CC_HAVE_STACKPROTECTOR_SYSREG
        def_bool $(cc-option,-mstack-protector-guard=sysreg -mstack-protector-guard-reg=sp_el0 -mstack-protector-guard-offset=0)

diff --git a/arch/arm64/kernel/kaslr.c b/arch/arm64/kernel/kaslr.c
index cfa2cfd..418b2bb 100644 (file)
--- a/arch/arm64/kernel/kaslr.c
+++ b/arch/arm64/kernel/kaslr.c
@@ -162,7 +162,9 @@ u64 __init kaslr_early_init(void)
                 * a PAGE_SIZE multiple in the range [_etext - MODULES_VSIZE,
                 * _stext) . This guarantees that the resulting region still
                 * covers [_stext, _etext], and that all relative branches can
-                * be resolved without veneers.
+                * be resolved without veneers unless this region is exhausted
+                * and we fall back to a larger 2GB window in module_alloc()
+                * when ARM64_MODULE_PLTS is enabled.
                 */
                module_range = MODULES_VSIZE - (u64)(_etext - _stext);
                module_alloc_base = (u64)_etext + offset - MODULES_VSIZE;