RDMA/bnxt_re: zero shared page before exposing to userspace

commit f6b079629b upstream.

bnxt_re_alloc_ucontext() allocates uctx->shpg via
__get_free_page(GFP_KERNEL). The buddy allocator does not zero pages
without __GFP_ZERO, so the page contains stale kernel data from
whatever object most recently freed it.

The page is then mapped into userspace via vm_insert_page() under
BNXT_RE_MMAP_SH_PAGE in bnxt_re_mmap(). The driver only ever writes
4 bytes (a u32 AVID) at offset BNXT_RE_AVID_OFFT (0x10) inside
bnxt_re_create_ah(); the remaining 4092 bytes of the page are exposed
to userspace unsanitised, leaking kernel memory contents.

Any user with access to /dev/infiniband/uverbsX on a host with a
bnxt_re device (typically rdma group membership) can read this data
via a single mmap() at pgoff 0 after IB_USER_VERBS_CMD_GET_CONTEXT.

Other shared pages in the same file already use get_zeroed_page()
correctly:

  drivers/infiniband/hw/bnxt_re/ib_verbs.c
      srq->uctx_srq_page = (void *)get_zeroed_page(GFP_KERNEL);
      cq->uctx_cq_page  = (void *)get_zeroed_page(GFP_KERNEL);

uctx->shpg is the only outlier. Bring it in line with the existing
convention by switching to get_zeroed_page().

Fixes: 1ac5a40479 ("RDMA/bnxt_re: Add bnxt_re RoCE driver")
Signed-off-by: Lord Ulf Henrik Holmberg <henrik.holmberg@defensify.se>
Link: https://patch.msgid.link/20260509084011.11971-1-pomzm67@gmail.com
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/infiniband/hw/bnxt_re/ib_verbs.c

@@ -4146,7 +4146,7 @@ int bnxt_re_alloc_ucontext(struct ib_ucontext *ctx, struct ib_udata *udata)
 
 	uctx->rdev = rdev;
 
-	uctx->shpg = (void *)__get_free_page(GFP_KERNEL);
+	uctx->shpg = (void *)get_zeroed_page(GFP_KERNEL);
 	if (!uctx->shpg) {
 		rc = -ENOMEM;
 		goto fail;