Eric Blake
2022-May-31 14:15 UTC
[Libguestfs] [libnbd PATCH 1/2] api: Tighter checking of structured read replies
Now that we allow clients to bypass buffer pre-initialization, it
becomes more important to detect when a buggy server using structured
replies does not send us enough bytes to cover the requested read
size. Our check is not perfect (a server that duplicates reply chunks
for byte 0 and omits byte 1 gets past our check), but this is a
tighter sanity check so that we are less likely to report a successful
read containing uninitialized memory on a buggy server.
Because we have a maximum read buffer size of 64M, and first check
that the server's chunk fits bounds, we don't have to worry about
overflowing a uint32_t, even if a server sends enough duplicate
responses that an actual sum would overflow.
---
lib/internal.h | 2 +-
generator/states-reply-simple.c | 4 ++--
generator/states-reply-structured.c | 6 ++++--
lib/aio.c | 7 +++++--
4 files changed, 12 insertions(+), 7 deletions(-)
diff --git a/lib/internal.h b/lib/internal.h
index 885cee1..4121a5c 100644
--- a/lib/internal.h
+++ b/lib/internal.h
@@ -352,8 +352,8 @@ struct command {
void *data; /* Buffer for read/write */
struct command_cb cb;
enum state state; /* State to resume with on next POLLIN */
- bool data_seen; /* For read, true if at least one data chunk seen */
bool initialized; /* For read, true if getting a hole may skip memset */
+ uint32_t data_seen; /* For read, cumulative size of data chunks seen */
uint32_t error; /* Local errno value */
};
diff --git a/generator/states-reply-simple.c b/generator/states-reply-simple.c
index 7dc26fd..2a7b9a9 100644
--- a/generator/states-reply-simple.c
+++ b/generator/states-reply-simple.c
@@ -1,5 +1,5 @@
/* nbd client library in userspace: state machine
- * Copyright (C) 2013-2019 Red Hat Inc.
+ * Copyright (C) 2013-2022 Red Hat Inc.
*
* This library is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
@@ -40,7 +40,7 @@ STATE_MACHINE {
if (cmd->error == 0 && cmd->type == NBD_CMD_READ) {
h->rbuf = cmd->data;
h->rlen = cmd->count;
- cmd->data_seen = true;
+ cmd->data_seen = cmd->count;
SET_NEXT_STATE (%RECV_READ_PAYLOAD);
}
else {
diff --git a/generator/states-reply-structured.c
b/generator/states-reply-structured.c
index 12c24f5..cabd543 100644
--- a/generator/states-reply-structured.c
+++ b/generator/states-reply-structured.c
@@ -354,7 +354,6 @@ STATE_MACHINE {
assert (cmd); /* guaranteed by CHECK */
assert (cmd->data && cmd->type == NBD_CMD_READ);
- cmd->data_seen = true;
/* Length of the data following. */
length -= 8;
@@ -364,6 +363,8 @@ STATE_MACHINE {
SET_NEXT_STATE (%.DEAD);
return 0;
}
+ if (cmd->data_seen <= cmd->count)
+ cmd->data_seen += length;
/* Now this is the byte offset in the read buffer. */
offset -= cmd->offset;
@@ -422,13 +423,14 @@ STATE_MACHINE {
assert (cmd); /* guaranteed by CHECK */
assert (cmd->data && cmd->type == NBD_CMD_READ);
- cmd->data_seen = true;
/* Is the data within bounds? */
if (! structured_reply_in_bounds (offset, length, cmd)) {
SET_NEXT_STATE (%.DEAD);
return 0;
}
+ if (cmd->data_seen <= cmd->count)
+ cmd->data_seen += length;
/* Now this is the byte offset in the read buffer. */
offset -= cmd->offset;
diff --git a/lib/aio.c b/lib/aio.c
index 9744840..dc01f90 100644
--- a/lib/aio.c
+++ b/lib/aio.c
@@ -1,5 +1,5 @@
/* NBD client library in userspace
- * Copyright (C) 2013-2019 Red Hat Inc.
+ * Copyright (C) 2013-2022 Red Hat Inc.
*
* This library is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
@@ -91,8 +91,11 @@ nbd_unlocked_aio_command_completed (struct nbd_handle *h,
assert (cmd->type != NBD_CMD_DISC);
/* The spec states that a 0-length read request is unspecified; but
* it is easy enough to treat it as successful as an extension.
+ * Conversely, make sure a server sending structured replies sent
+ * enough data chunks to cover the overall count (although we do not
+ * detect if it duplicated some bytes while omitting others).
*/
- if (type == NBD_CMD_READ && !cmd->data_seen &&
cmd->count && !error)
+ if (type == NBD_CMD_READ && cmd->data_seen != cmd->count
&& !error)
error = EIO;
/* Retire it from the list and free it. */
--
2.36.1
Nir Soffer
2022-May-31 15:59 UTC
[Libguestfs] [libnbd PATCH 1/2] api: Tighter checking of structured read replies
On Tue, May 31, 2022 at 5:15 PM Eric Blake <eblake at redhat.com> wrote:> > Now that we allow clients to bypass buffer pre-initialization, it > becomes more important to detect when a buggy server using structured > replies does not send us enough bytes to cover the requested read > size. Our check is not perfect (a server that duplicates reply chunks > for byte 0 and omits byte 1 gets past our check), but this is a > tighter sanity check so that we are less likely to report a successful > read containing uninitialized memory on a buggy server.Nice!> Because we have a maximum read buffer size of 64M, and first check > that the server's chunk fits bounds, we don't have to worry about > overflowing a uint32_t, even if a server sends enough duplicate > responses that an actual sum would overflow. > --- > lib/internal.h | 2 +- > generator/states-reply-simple.c | 4 ++-- > generator/states-reply-structured.c | 6 ++++-- > lib/aio.c | 7 +++++-- > 4 files changed, 12 insertions(+), 7 deletions(-) > > diff --git a/lib/internal.h b/lib/internal.h > index 885cee1..4121a5c 100644 > --- a/lib/internal.h > +++ b/lib/internal.h > @@ -352,8 +352,8 @@ struct command { > void *data; /* Buffer for read/write */ > struct command_cb cb; > enum state state; /* State to resume with on next POLLIN */ > - bool data_seen; /* For read, true if at least one data chunk seen */ > bool initialized; /* For read, true if getting a hole may skip memset */ > + uint32_t data_seen; /* For read, cumulative size of data chunks seen */ > uint32_t error; /* Local errno value */ > }; > > diff --git a/generator/states-reply-simple.c b/generator/states-reply-simple.c > index 7dc26fd..2a7b9a9 100644 > --- a/generator/states-reply-simple.c > +++ b/generator/states-reply-simple.c > @@ -1,5 +1,5 @@ > /* nbd client library in userspace: state machine > - * Copyright (C) 2013-2019 Red Hat Inc. > + * Copyright (C) 2013-2022 Red Hat Inc. > * > * This library is free software; you can redistribute it and/or > * modify it under the terms of the GNU Lesser General Public > @@ -40,7 +40,7 @@ STATE_MACHINE { > if (cmd->error == 0 && cmd->type == NBD_CMD_READ) { > h->rbuf = cmd->data; > h->rlen = cmd->count; > - cmd->data_seen = true; > + cmd->data_seen = cmd->count; > SET_NEXT_STATE (%RECV_READ_PAYLOAD); > } > else { > diff --git a/generator/states-reply-structured.c b/generator/states-reply-structured.c > index 12c24f5..cabd543 100644 > --- a/generator/states-reply-structured.c > +++ b/generator/states-reply-structured.c > @@ -354,7 +354,6 @@ STATE_MACHINE { > assert (cmd); /* guaranteed by CHECK */ > > assert (cmd->data && cmd->type == NBD_CMD_READ); > - cmd->data_seen = true; > > /* Length of the data following. */ > length -= 8; > @@ -364,6 +363,8 @@ STATE_MACHINE { > SET_NEXT_STATE (%.DEAD); > return 0; > } > + if (cmd->data_seen <= cmd->count) > + cmd->data_seen += length;This does not feel right. if you received more data, it should be counted, and if this causes data_seen to be bigger than cmd->count, isn't this a fatal error?> /* Now this is the byte offset in the read buffer. */ > offset -= cmd->offset; > > @@ -422,13 +423,14 @@ STATE_MACHINE { > assert (cmd); /* guaranteed by CHECK */ > > assert (cmd->data && cmd->type == NBD_CMD_READ); > - cmd->data_seen = true; > > /* Is the data within bounds? */ > if (! structured_reply_in_bounds (offset, length, cmd)) { > SET_NEXT_STATE (%.DEAD); > return 0; > } > + if (cmd->data_seen <= cmd->count) > + cmd->data_seen += length;Same here> /* Now this is the byte offset in the read buffer. */ > offset -= cmd->offset; > > diff --git a/lib/aio.c b/lib/aio.c > index 9744840..dc01f90 100644 > --- a/lib/aio.c > +++ b/lib/aio.c > @@ -1,5 +1,5 @@ > /* NBD client library in userspace > - * Copyright (C) 2013-2019 Red Hat Inc. > + * Copyright (C) 2013-2022 Red Hat Inc. > * > * This library is free software; you can redistribute it and/or > * modify it under the terms of the GNU Lesser General Public > @@ -91,8 +91,11 @@ nbd_unlocked_aio_command_completed (struct nbd_handle *h, > assert (cmd->type != NBD_CMD_DISC); > /* The spec states that a 0-length read request is unspecified; but > * it is easy enough to treat it as successful as an extension. > + * Conversely, make sure a server sending structured replies sent > + * enough data chunks to cover the overall count (although we do not > + * detect if it duplicated some bytes while omitting others). > */ > - if (type == NBD_CMD_READ && !cmd->data_seen && cmd->count && !error) > + if (type == NBD_CMD_READ && cmd->data_seen != cmd->count && !error) > error = EIO; > > /* Retire it from the list and free it. */ > -- > 2.36.1 > > _______________________________________________ > Libguestfs mailing list > Libguestfs at redhat.com > https://listman.redhat.com/mailman/listinfo/libguestfsNir