On Jun 28, 2019, at 12:41 PM, Yu-cheng Yu [off-list ref] wrote:

The CET legacy code bitmap covers the whole user-mode address space and is
located at the top of the user-mode address space.  It is allocated only
when the first time arch_prctl(ARCH_X86_MARK_LEGACY_CODE) is called from
an application.

Introduce:

arch_prctl(ARCH_X86_MARK_LEGACY_CODE, unsigned long *buf)
   Mark an address range as IBT legacy code.

How about defining a struct for this?

The change log should discuss where the bitmap goes and how it’s allocated.

+static int alloc_bitmap(void)
+{
+    unsigned long addr;
+    u64 msr_ia32_u_cet;
+
+    addr = do_mmap_locked(NULL, IBT_BITMAP_ADDR, IBT_BITMAP_SIZE,
+                  PROT_READ | PROT_WRITE,
+                  MAP_ANONYMOUS | MAP_PRIVATE | MAP_FIXED_NOREPLACE,
+                  VM_IBT | VM_NORESERVE, NULL);
+
+    if (IS_ERR((void *)addr))
+        return addr;
+
+    current->thread.cet.ibt_bitmap_addr = addr;

addr is a constant. Why are you storing it?  If it ends up not being
constant, you should wire up mremap like the vDSO does.

+static int set_user_bits(unsigned long __user *buf, unsigned long buf_size,
+             unsigned long start_bit, unsigned long end_bit, unsigned long set)
+{
+    unsigned long start_ul, end_ul, total_ul;
+    int i, j, r;
+
+    if (round_up(end_bit, BITS_PER_BYTE) / BITS_PER_BYTE > buf_size)
+        end_bit = buf_size * BITS_PER_BYTE - 1;
+
+    start_ul = start_bit / BITS_PER_LONG;
+    end_ul = end_bit / BITS_PER_LONG;
+    total_ul = (end_ul - start_ul + 1);
+
+    i = start_bit % BITS_PER_LONG;
+    j = end_bit % BITS_PER_LONG;
+
+    r = 0;
+    put_user_try {

put_user_try is obsolete.  Just use get_user(), etc.

Also, I must be missing something fundamental, because this series
claims that user code can't write directly to the bitmap.  This means
that this entire function shouldn't work at all.