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

[Will Deacon]

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@xxxxxxx> 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.