Re: [PATCH v3 20/20] arm64: kaslr: Put kernel vectors address in separate data page

Will Deacon
2017-12-06T13:27:11Z

Hi Ard,

On Wed, Dec 06, 2017 at 12:59:40PM +0000, Ard Biesheuvel wrote:
> On 6 December 2017 at 12:35, Will Deacon <will.deacon@arm.com> wrote:
> > The literal pool entry for identifying the vectors base is the only piece
> > of information in the trampoline page that identifies the true location
> > of the kernel.
> >
> > This patch moves it into its own page, which is only mapped by the full
> > kernel page table, which protects against any accidental leakage of the
> > trampoline contents.

[...]

> > @@ -1073,6 +1079,11 @@ END(tramp_exit_compat)
> >
> > .ltorg
> > .popsection // .entry.tramp.text
> > +#ifdef CONFIG_RANDOMIZE_BASE
> > + .pushsection ".entry.tramp.data", "a" // .entry.tramp.data
> > + .quad vectors
> > + .popsection // .entry.tramp.data
>
> This does not need to be in a section of its own, and doesn't need to
> be padded to a full page.
>
> If you just stick this in .rodata but align it to page size, you can
> just map whichever page it ends up in into the TRAMP_DATA fixmap slot
> (which is a r/o mapping anyway). You could then drop most of the
> changes below. And actually, I'm not entirely sure whether it still
> makes sense then to do this only for CONFIG_RANDOMIZE_BASE.

Good point; I momentarily forgot this was in the kernel page table anyway.
How about something like the diff below merged on top (so this basically
undoes a bunch of the patch)?

I'd prefer to keep the CONFIG_RANDOMIZE_BASE dependency, at least for now.

Will

--->8

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index a70c6dd2cc19..031392ee5f47 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -1080,9 +1080,12 @@ END(tramp_exit_compat)
.ltorg
.popsection // .entry.tramp.text
#ifdef CONFIG_RANDOMIZE_BASE
- .pushsection ".entry.tramp.data", "a" // .entry.tramp.data
+ .pushsection ".rodata", "a"
+ .align PAGE_SHIFT
+ .globl __entry_tramp_data_start
+__entry_tramp_data_start:
.quad vectors
- .popsection // .entry.tramp.data
+ .popsection // .rodata
#endif /* CONFIG_RANDOMIZE_BASE */
#endif /* CONFIG_UNMAP_KERNEL_AT_EL0 */

diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
index 976109b3ae51..27cf9be20a00 100644
--- a/arch/arm64/kernel/vmlinux.lds.S
+++ b/arch/arm64/kernel/vmlinux.lds.S
@@ -64,21 +64,8 @@ jiffies = jiffies_64;
*(.entry.tramp.text) \
. = ALIGN(PAGE_SIZE); \
VMLINUX_SYMBOL(__entry_tramp_text_end) = .;
-#ifdef CONFIG_RANDOMIZE_BASE
-#define TRAMP_DATA \
- .entry.tramp.data : { \
- . = ALIGN(PAGE_SIZE); \
- VMLINUX_SYMBOL(__entry_tramp_data_start) = .; \
- *(.entry.tramp.data) \
- . = ALIGN(PAGE_SIZE); \
- VMLINUX_SYMBOL(__entry_tramp_data_end) = .; \
- }
-#else
-#define TRAMP_DATA
-#endif /* CONFIG_RANDOMIZE_BASE */
#else
#define TRAMP_TEXT
-#define TRAMP_DATA
#endif /* CONFIG_UNMAP_KERNEL_AT_EL0 */

/*
@@ -150,7 +137,6 @@ SECTIONS
RO_DATA(PAGE_SIZE) /* everything from this point to */
EXCEPTION_TABLE(8) /* __init_begin will be marked RO NX */
NOTES
- TRAMP_DATA

. = ALIGN(SEGMENT_ALIGN);
__init_begin = .;
@@ -268,10 +254,6 @@ ASSERT(__hibernate_exit_text_end - (__hibernate_exit_text_start & ~(SZ_4K - 1))
#ifdef CONFIG_UNMAP_KERNEL_AT_EL0
ASSERT((__entry_tramp_text_end - __entry_tramp_text_start) == PAGE_SIZE,
"Entry trampoline text too big")
-#ifdef CONFIG_RANDOMIZE_BASE
-ASSERT((__entry_tramp_data_end - __entry_tramp_data_start) == PAGE_SIZE,
- "Entry trampoline data too big")
-#endif
#endif
/*
* If padding is applied before .head.text, virt<->phys conversions will fail.