From: Nguyễn Thái Ngọc Duy <pclouds@xxxxxxxxx>
There are "holes" in the index-helper approach because the shared
memory is not verified again by git. If $USER is compromised, shared
memory could be modified. But anyone who could do this could already
modify $GIT_DIR/index. A more realistic risk is some bugs in
index-helper that produce corrupt shared memory. --strict is added to
avoid that.
Strictly speaking there's still a very small gap where corrupt shared
memory could still be read by git: after we write the trailing SHA-1 in
the shared memory (thus signaling "this shm is ready") and before
verify_shm() detects an error.
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@xxxxxxxxx>
Signed-off-by: David Turner <dturner@xxxxxxxxxxxxxxxx>
---
Documentation/git-index-helper.txt | 9 +++++++
cache.h | 1 +
index-helper.c | 48 ++++++++++++++++++++++++++++++++++++++
read-cache.c | 9 ++++---
4 files changed, 64 insertions(+), 3 deletions(-)
diff --git a/Documentation/git-index-helper.txt b/Documentation/git-index-helper.txt
index 77687c0..a5c058f 100644
--- a/Documentation/git-index-helper.txt
+++ b/Documentation/git-index-helper.txt
@@ -22,6 +22,15 @@ OPTIONS
Exit if the cached index is not accessed for `<n>`
seconds. Specify 0 to wait forever. Default is 600.
+--strict::
+--no-strict::
+ Strict mode makes index-helper verify the shared memory after
+ it's created. If the result does not match what's read from
+ $GIT_DIR/index, the shared memory is destroyed. This makes
+ index-helper take more than double the amount of time required
+ for reading an index, but because it will happen in the
+ background, it's not noticable. `--strict` is enabled by default.
+
NOTES
-----
diff --git a/cache.h b/cache.h
index 43fb314..4b678e9 100644
--- a/cache.h
+++ b/cache.h
@@ -336,6 +336,7 @@ struct index_state {
keep_mmap : 1,
from_shm : 1,
to_shm : 1,
+ always_verify_trailing_sha1 : 1,
initialized : 1;
struct hashmap name_hash;
struct hashmap dir_hash;
diff --git a/index-helper.c b/index-helper.c
index d64e49b..e8d6b64 100644
--- a/index-helper.c
+++ b/index-helper.c
@@ -16,6 +16,7 @@ struct shm {
static struct shm shm_index;
static struct shm shm_base_index;
+static int to_verify = 1;
static void release_index_shm(struct shm *is)
{
@@ -110,11 +111,56 @@ static void share_index(struct index_state *istate, struct shm *is)
hashcpy((unsigned char *)new_mmap + istate->mmap_size - 20, is->sha1);
}
+static int verify_shm(void)
+{
+ int i;
+ struct index_state istate;
+ memset(&istate, 0, sizeof(istate));
+ istate.always_verify_trailing_sha1 = 1;
+ istate.to_shm = 1;
+ i = read_index(&istate);
+ if (i != the_index.cache_nr)
+ goto done;
+ for (; i < the_index.cache_nr; i++) {
+ struct cache_entry *base, *ce;
+ /* namelen is checked separately */
+ const unsigned int ondisk_flags =
+ CE_STAGEMASK | CE_VALID | CE_EXTENDED_FLAGS;
+ unsigned int ce_flags, base_flags, ret;
+ base = the_index.cache[i];
+ ce = istate.cache[i];
+ if (ce->ce_namelen != base->ce_namelen ||
+ strcmp(ce->name, base->name)) {
+ warning("mismatch at entry %d", i);
+ break;
+ }
+ ce_flags = ce->ce_flags;
+ base_flags = base->ce_flags;
+ /* only on-disk flags matter */
+ ce->ce_flags &= ondisk_flags;
+ base->ce_flags &= ondisk_flags;
+ ret = memcmp(&ce->ce_stat_data, &base->ce_stat_data,
+ offsetof(struct cache_entry, name) -
+ offsetof(struct cache_entry, ce_stat_data));
+ ce->ce_flags = ce_flags;
+ base->ce_flags = base_flags;
+ if (ret) {
+ warning("mismatch at entry %d", i);
+ break;
+ }
+ }
+done:
+ discard_index(&istate);
+ return i == the_index.cache_nr;
+}
+
static void share_the_index(void)
{
if (the_index.split_index && the_index.split_index->base)
share_index(the_index.split_index->base, &shm_base_index);
share_index(&the_index, &shm_index);
+ if (to_verify && !verify_shm())
+ cleanup_shm();
discard_index(&the_index);
}
@@ -246,6 +292,8 @@ int main(int argc, char **argv)
struct option options[] = {
OPT_INTEGER(0, "exit-after", &idle_in_seconds,
N_("exit if not used after some seconds")),
+ OPT_BOOL(0, "strict", &to_verify,
+ "verify shared memory after creating"),
OPT_END()
};
diff --git a/read-cache.c b/read-cache.c
index af2101f..5df5b21 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -1669,9 +1669,12 @@ int do_read_index(struct index_state *istate, const char *path, int must_exist)
istate->mmap = mmap;
istate->mmap_size = mmap_size;
- if (try_shm(istate) &&
- verify_hdr(istate->mmap, istate->mmap_size) < 0)
- goto unmap;
+ if (try_shm(istate)) {
+ if (verify_hdr(istate->mmap, istate->mmap_size) < 0)
+ goto unmap;
+ } else if (istate->always_verify_trailing_sha1 &&
+ verify_hdr(istate->mmap, istate->mmap_size) < 0)
+ goto unmap;
hdr = mmap = istate->mmap;
mmap_size = istate->mmap_size;
if (!istate->keep_mmap)
--
2.4.2.767.g62658d5-twtrsrc
--
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