On Fri, 2018-10-05 at 09:28 -0700, Andy Lutomirski wrote:

quoted

On Oct 5, 2018, at 9:13 AM, Yu-cheng Yu [off-list ref] wrote:

quoted

On Wed, 2018-10-03 at 21:57 +0200, Eugene Syromiatnikov wrote:

quoted

On Fri, Sep 21, 2018 at 08:05:47AM -0700, Yu-cheng Yu wrote:
Indirect branch tracking provides an optional legacy code bitmap
that indicates locations of non-IBT compatible code.  When set,
each bit in the bitmap represents a page in the linear address is
legacy code.

We allocate the bitmap only when the application requests it.
Most applications do not need the bitmap.

Signed-off-by: Yu-cheng Yu <redacted>
---
arch/x86/kernel/cet.c | 45 +++++++++++++++++++++++++++++++++++++++++++
1 file changed, 45 insertions(+)

diff --git a/arch/x86/kernel/cet.c b/arch/x86/kernel/cet.c
index 6adfe795d692..a65d9745af08 100644
--- a/arch/x86/kernel/cet.c
+++ b/arch/x86/kernel/cet.c

@@ -314,3 +314,48 @@ void cet_disable_ibt(void)
   wrmsrl(MSR_IA32_U_CET, r);
   current->thread.cet.ibt_enabled = 0;

}
+
+int cet_setup_ibt_bitmap(void)
+{
+    u64 r;
+    unsigned long bitmap;
+    unsigned long size;
+
+    if (!cpu_feature_enabled(X86_FEATURE_IBT))
+        return -EOPNOTSUPP;
+
+    if (!current->thread.cet.ibt_bitmap_addr) {
+        /*
+         * Calculate size and put in thread header.
+         * may_expand_vm() needs this information.
+         */
+        size = TASK_SIZE / PAGE_SIZE / BITS_PER_BYTE;

TASK_SIZE_MAX is likely needed here, as an application can easily switch
between long an 32-bit protected mode.  And then the case of a CPU that
doesn't support 5LPT.

If we had calculated bitmap size from TASK_SIZE_MAX, all 32-bit apps would
have
failed the allocation for bitmap size > TASK_SIZE.  Please see values below,
which is printed from the current code.

Yu-cheng


x64:
TASK_SIZE_MAX    = 0000 7fff ffff f000
TASK_SIZE    = 0000 7fff ffff f000
bitmap size    = 0000 0000 ffff ffff

x32:
TASK_SIZE_MAX    = 0000 7fff ffff f000
TASK_SIZE    = 0000 0000 ffff e000
bitmap size    = 0000 0000 0001 ffff

I haven’t followed all the details here, but I have a general policy of
objecting to any new use of TASK_SIZE. If you really really need to depend on
32-bitness in new code, please figure out what exactly you mean by “32-bit”
and use an explicit check.

The explicit check would be:

test_thread_flag(TIF_ADDR32) ? IA32_PAGE_OFFSET : TASK_SIZE_MAX

which is the same as TASK_SIZE.

Or, do we want a new macro?

#define IBT_BITMAP_SIZE (test_thread_flag(TIF_ADDR32) ? \
	(IA32_PAGE_OFFSET / PAGE_SIZE / BITS_PER_BYTE) : \
	(TASK_SIZE_MAX / PAGE_SIZE / BITS_PER_BYTE))

Yu-cheng