simple-ipc/ipc-win32: add Windows ACL to named pipe
Set an ACL on the named pipe to allow the well-known group EVERYONE to read and write to the IPC server's named pipe. In the event that the daemon was started with elevation, allow non-elevated clients to communicate with the daemon. Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
Jeff Hostetler committed
Sep 20, 2021 at 15:36 UTC
8750249053c6441e912a41a2feca8f22f73babc4
1 file changed
+129
-11
compat/simple-ipc/ipc-win32.c
+129
-11
@@ -3,6 +3,8 @@
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#include "strbuf.h"
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#include "pkt-line.h"
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#include "thread-utils.h"
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+#include "accctrl.h"
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+#include "aclapi.h"
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#ifndef SUPPORTS_SIMPLE_IPC
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/*
@@ -592,11 +594,132 @@ finished:
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return NULL;
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}
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+/*
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+ * We need to build a Windows "SECURITY_ATTRIBUTES" object and use it
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+ * to apply an ACL when we create the initial instance of the Named
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+ * Pipe. The construction is somewhat involved and consists of
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+ * several sequential steps and intermediate objects.
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+ *
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+ * We use this structure to hold these intermediate pointers so that
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+ * we can free them as a group. (It is unclear from the docs whether
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+ * some of these intermediate pointers can be freed before we are
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+ * finished using the "lpSA" member.)
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+ */
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+struct my_sa_data
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+{
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+ PSID pEveryoneSID;
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+ PACL pACL;
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+ PSECURITY_DESCRIPTOR pSD;
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+ LPSECURITY_ATTRIBUTES lpSA;
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+};
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+
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+static void init_sa(struct my_sa_data *d)
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+{
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+ memset(d, 0, sizeof(*d));
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+}
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+
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+static void release_sa(struct my_sa_data *d)
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+{
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+ if (d->pEveryoneSID)
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+ FreeSid(d->pEveryoneSID);
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+ if (d->pACL)
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+ LocalFree(d->pACL);
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+ if (d->pSD)
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+ LocalFree(d->pSD);
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+ if (d->lpSA)
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+ LocalFree(d->lpSA);
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+
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+ memset(d, 0, sizeof(*d));
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+}
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+
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+/*
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+ * Create SECURITY_ATTRIBUTES to apply to the initial named pipe. The
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+ * creator of the first server instance gets to set the ACLs on it.
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+ *
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+ * We allow the well-known group `EVERYONE` to have read+write access
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+ * to the named pipe so that clients can send queries to the daemon
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+ * and receive the response.
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+ *
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+ * Normally, this is not necessary since the daemon is usually
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+ * automatically started by a foreground command like `git status`,
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+ * but in those cases where an elevated Git command started the daemon
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+ * (such that the daemon itself runs with elevation), we need to add
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+ * the ACL so that non-elevated commands can write to it.
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+ *
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+ * The following document was helpful:
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+ * https://docs.microsoft.com/en-us/windows/win32/secauthz/creating-a-security-descriptor-for-a-new-object-in-c--
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+ *
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+ * Returns d->lpSA set to a SA or NULL.
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+ */
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+static LPSECURITY_ATTRIBUTES get_sa(struct my_sa_data *d)
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+{
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+ SID_IDENTIFIER_AUTHORITY sid_auth_world = SECURITY_WORLD_SID_AUTHORITY;
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+#define NR_EA (1)
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+ EXPLICIT_ACCESS ea[NR_EA];
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+ DWORD dwResult;
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+
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+ if (!AllocateAndInitializeSid(&sid_auth_world, 1,
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+ SECURITY_WORLD_RID, 0,0,0,0,0,0,0,
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+ &d->pEveryoneSID)) {
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+ DWORD gle = GetLastError();
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+ trace2_data_intmax("ipc-debug", NULL, "alloc-world-sid/gle",
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+ (intmax_t)gle);
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+ goto fail;
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+ }
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+
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+ memset(ea, 0, NR_EA * sizeof(EXPLICIT_ACCESS));
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+
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+ ea[0].grfAccessPermissions = GENERIC_READ | GENERIC_WRITE;
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+ ea[0].grfAccessMode = SET_ACCESS;
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+ ea[0].grfInheritance = NO_INHERITANCE;
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+ ea[0].Trustee.MultipleTrusteeOperation = NO_MULTIPLE_TRUSTEE;
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+ ea[0].Trustee.TrusteeForm = TRUSTEE_IS_SID;
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+ ea[0].Trustee.TrusteeType = TRUSTEE_IS_WELL_KNOWN_GROUP;
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+ ea[0].Trustee.ptstrName = (LPTSTR)d->pEveryoneSID;
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+
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+ dwResult = SetEntriesInAcl(NR_EA, ea, NULL, &d->pACL);
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+ if (dwResult != ERROR_SUCCESS) {
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+ DWORD gle = GetLastError();
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+ trace2_data_intmax("ipc-debug", NULL, "set-acl-entry/gle",
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+ (intmax_t)gle);
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+ trace2_data_intmax("ipc-debug", NULL, "set-acl-entry/dw",
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+ (intmax_t)dwResult);
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+ goto fail;
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+ }
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+
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+ d->pSD = (PSECURITY_DESCRIPTOR)LocalAlloc(
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+ LPTR, SECURITY_DESCRIPTOR_MIN_LENGTH);
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+ if (!InitializeSecurityDescriptor(d->pSD, SECURITY_DESCRIPTOR_REVISION)) {
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+ DWORD gle = GetLastError();
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+ trace2_data_intmax("ipc-debug", NULL, "init-sd/gle", (intmax_t)gle);
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+ goto fail;
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+ }
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+
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+ if (!SetSecurityDescriptorDacl(d->pSD, TRUE, d->pACL, FALSE)) {
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+ DWORD gle = GetLastError();
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+ trace2_data_intmax("ipc-debug", NULL, "set-sd-dacl/gle", (intmax_t)gle);
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+ goto fail;
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+ }
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+
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+ d->lpSA = (LPSECURITY_ATTRIBUTES)LocalAlloc(LPTR, sizeof(SECURITY_ATTRIBUTES));
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+ d->lpSA->nLength = sizeof(SECURITY_ATTRIBUTES);
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+ d->lpSA->lpSecurityDescriptor = d->pSD;
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+ d->lpSA->bInheritHandle = FALSE;
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+
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+ return d->lpSA;
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+
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+fail:
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+ release_sa(d);
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+ return NULL;
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+}
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+
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static HANDLE create_new_pipe(wchar_t *wpath, int is_first)
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{
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HANDLE hPipe;
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DWORD dwOpenMode, dwPipeMode;
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- LPSECURITY_ATTRIBUTES lpsa = NULL;
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+ struct my_sa_data my_sa_data;
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+
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+ init_sa(&my_sa_data);
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dwOpenMode = PIPE_ACCESS_INBOUND | PIPE_ACCESS_OUTBOUND |
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FILE_FLAG_OVERLAPPED;
@@ -612,20 +735,15 @@ static HANDLE create_new_pipe(wchar_t *wpath, int is_first)
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* set the ACL / Security Attributes on the named
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* pipe; subsequent instances inherit and cannot
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* change them.
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- *
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- * TODO Should we allow the application layer to
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- * specify security attributes, such as `LocalService`
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- * or `LocalSystem`, when we create the named pipe?
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- * This question is probably not important when the
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- * daemon is started by a foreground user process and
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- * only needs to talk to the current user, but may be
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- * if the daemon is run via the Control Panel as a
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- * System Service.
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*/
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+ get_sa(&my_sa_data);
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}
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hPipe = CreateNamedPipeW(wpath, dwOpenMode, dwPipeMode,
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- PIPE_UNLIMITED_INSTANCES, 1024, 1024, 0, lpsa);
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+ PIPE_UNLIMITED_INSTANCES, 1024, 1024, 0,
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+ my_sa_data.lpSA);
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+
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+ release_sa(&my_sa_data);
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return hPipe;
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}