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From cdee6bd650a35075515d4fe2bb67657811c9640c Mon Sep 17 00:00:00 2001
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From: Joe Lawrence <joe.lawrence@redhat.com>
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Date: Mon, 16 Nov 2020 15:21:59 -0500
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Subject: [PATCH] kpatch: wait for module ref counts on unload
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There exists a very small timing window in which "kpatch unload" gets to
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its "rmmod" step before the kpatch-patch module's reference count has
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cleared and the "rmmod" fails.
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This is only a transient problem, but we can adopt code from upstream
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livepatch kselftests which wait for the module refcounts to settle
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before moving onto "rmmod".
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A small wrinkle is that this is not supported by the older kpatch.ko
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core. The price for circumventing the activeness safety check via
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KPATCH_FORCE_UNSAFE is that it must leave the kpatch patch modules in
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place (see e1890e627a9b ("prevent rmmod of forced modules")).
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Signed-off-by: Joe Lawrence <joe.lawrence@redhat.com>
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---
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kpatch/kpatch | 40 ++++++++++++++++++++++++++++++++++++++--
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1 file changed, 38 insertions(+), 2 deletions(-)
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diff --git a/kpatch/kpatch b/kpatch/kpatch
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index bca8f41..b35b742 100755
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--- a/kpatch/kpatch
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+++ b/kpatch/kpatch
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@@ -28,6 +28,7 @@ SCRIPTDIR="$(readlink -f "$(dirname "$(type -p "$0")")")"
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VERSION="0.9.2"
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POST_ENABLE_WAIT=15 # seconds
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POST_SIGNAL_WAIT=60 # seconds
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+MODULE_REF_WAIT=15 # seconds
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# How many times to try loading the patch if activeness safety check fails.
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MAX_LOAD_ATTEMPTS=5
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@@ -125,6 +126,10 @@ find_core_module() {
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return 1
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}
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+kpatch_core_loaded() {
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+ grep -q -e "T kpatch_register" /proc/kallsyms
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+}
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+
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core_loaded () {
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grep -q -e "T klp_enable_patch" -e "T kpatch_register" /proc/kallsyms
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}
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@@ -265,6 +270,31 @@ wait_for_patch_transition() {
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return 1
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}
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+module_ref_count() {
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+ local modname="$1"
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+ [[ $(cat "/sys/module/$modname/refcnt" 2>/dev/null) != "0" ]]
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+}
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+
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+wait_for_zero_module_ref_count() {
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+ local modname="$1"
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+ local i=0
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+
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+ # We can't rely on a zero refcount with kpatch.ko as it
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+ # implements KPATCH_FORCE_UNSAFE with an additional reference on
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+ # kpatch-patch modules to avoid potential crashes.
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+ kpatch_core_loaded && return 0
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+
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+ module_ref_count "$modname" || return 0
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+
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+ echo "waiting (up to $MODULE_REF_WAIT seconds) for module refcount..."
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+ for (( i=0; i
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+ module_ref_count "$modname" || return 0
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+ sleep 1s
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+ done
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+
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+ return 1
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+}
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+
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load_module () {
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local module="$1"
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@@ -381,10 +411,16 @@ disable_patch_strict () {
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}
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remove_module () {
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- echo "unloading patch module: $1"
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+ local modname="$1"
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+
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+ if ! wait_for_zero_module_ref_count "$modname"; then
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+ die "failed to unload module $modname (refcnt)"
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+ fi
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+
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+ echo "unloading patch module: $modname"
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# ignore any error here because rmmod can fail if the module used
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# KPATCH_FORCE_UNSAFE.
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- rmmod "$1" 2> /dev/null || return 0
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+ rmmod "$modname" 2> /dev/null || return 0
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}
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unload_module () {
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--
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2.25.4
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