[PATCH v2] nvmet: fix pre-auth out-of-bounds heap read in Discovery Get Log Page

[hexlabsecurity at proton.me]

>From 6710e68439c458d691a4fe5c7fa354404745dd0a Mon Sep 17 00:00:00 2001
From: Bryam Vargas <hexlabsecurity at proton.me>
Date: Wed, 27 May 2026 15:00:00 -0500
Subject: [PATCH v2] nvmet: fix pre-auth out-of-bounds heap read in Discovery
 Get Log Page

nvmet_execute_disc_get_log_page() validates only the dword alignment
of the host-supplied Log Page Offset (lpo).  The 64-bit offset is then
added to a small kzalloc'd buffer that holds the discovery log page
and the result is passed straight to nvmet_copy_to_sgl(), which
memcpy()s data_len bytes out to the host with no source-side bound
check:

    u64 offset      = nvmet_get_log_page_offset(req->cmd);  /* 64-bit host */
    size_t data_len = nvmet_get_log_page_len(req->cmd);     /* 32-bit host */
    ...
    if (offset & 0x3) { ... }                               /* only check */
    ...
    alloc_len = sizeof(*hdr) + entry_size * discovery_log_entries(req);
    buffer = kzalloc(alloc_len, GFP_KERNEL);
    ...
    status = nvmet_copy_to_sgl(req, 0, buffer + offset, data_len);

The Discovery controller is unauthenticated -- nvmet_host_allowed()
returns true unconditionally for the discovery subsystem -- so the call
is reachable pre-authentication by any TCP/RDMA/FC peer that can reach
the nvmet target.  With a discovery log page of ~1 KiB, an attacker
requesting up to 4 KiB starting at offset == alloc_len reads the next
slab page out and gets its content returned over the fabric (an
empirical run on a default nvmet-tcp loopback target leaked 81
canonical kernel pointers in one Get Log Page response).  Pointing the
offset at unmapped kernel memory faults the in-kernel memcpy and
crashes (or panics, on panic_on_oops=1) the target host instead.

The attacker-controlled source-side offset pattern
"nvmet_copy_to_sgl(req, 0, buffer + ATTACKER_OFFSET, ...)" is unique
to nvmet_execute_disc_get_log_page in the entire nvmet codebase: every
other Get Log Page handler in admin-cmd.c either ignores lpo (and
silently starts every response at offset 0) or tracks a local
destination offset with a fixed source pointer.

Validate the host-supplied offset against the log page size, cap the
copy length to what is actually available, and zero-fill any remainder
of the host transfer buffer.  The zero-fill matches the existing
short-response pattern in nvmet_execute_get_log_changed_ns()
(admin-cmd.c) and prevents leaking transport SGL contents when the
host asks for more bytes than the log page contains.

Reported-by: Bryam Vargas <hexlabsecurity at proton.me>
Suggested-by: Christoph Hellwig <hch at lst.de>
Fixes: a07b4970f464 ("nvmet: add a generic NVMe target")
Cc: stable at vger.kernel.org
Signed-off-by: Bryam Vargas <hexlabsecurity at proton.me>
---
v2: rewrote the validation flow per Christoph's suggestion -- single
    `out_free_buffer` cleanup label reached by `goto` on the offset
    overflow path, `min_t(size_t, ...)` for the capped copy length,
    one fewer level of nesting.  `min_t(size_t, ...)` (rather than
    bare `min()`) because `data_len` is `size_t` and `alloc_len -
    offset` promotes to `unsigned long long` (since `offset` is
    `u64`), which trips the kernel min() __typecheck; the size_t
    rendition matches the analogous shape in io-cmd-bdev.c:228.

    Empirically verified on a Linux 6.12.90 KASAN INLINE +
    kasan.fault=panic VM: pre-fix `nvme get-log lpo=alloc_len
    len=4096` reboots the host via KASAN catching the OOB memcpy;
    post-fix the same probe returns cleanly with zero kernel-pointer
    qwords leaked, and `lpo > alloc_len` returns
    NVME_SC_INVALID_FIELD as intended.

v1: https://lore.kernel.org/linux-nvme/ -- (search by Reported-by)

 drivers/nvme/target/discovery.c | 23 ++++++++++++++++++++++-
 1 file changed, 22 insertions(+), 1 deletion(-)

diff --git a/drivers/nvme/target/discovery.c b/drivers/nvme/target/discovery.c
index e9b35549e254..114869d16a1f 100644
--- a/drivers/nvme/target/discovery.c
+++ b/drivers/nvme/target/discovery.c
@@ -166,6 +166,7 @@ static void nvmet_execute_disc_get_log_page(struct nvmet_req *req)
 	u64 offset = nvmet_get_log_page_offset(req->cmd);
 	size_t data_len = nvmet_get_log_page_len(req->cmd);
 	size_t alloc_len;
+	size_t copy_len;
 	struct nvmet_subsys_link *p;
 	struct nvmet_port *r;
 	u32 numrec = 0;
@@ -242,7 +243,27 @@ static void nvmet_execute_disc_get_log_page(struct nvmet_req *req)
 
 	up_read(&nvmet_config_sem);
 
-	status = nvmet_copy_to_sgl(req, 0, buffer + offset, data_len);
+	/*
+	 * Validate the host-supplied log page offset before copying out.
+	 * Without this check, the host controls a 64-bit byte offset into
+	 * a small kzalloc'd buffer: a value past the log page lets the
+	 * subsequent memcpy read adjacent kernel heap, and a value aimed
+	 * at unmapped kernel memory faults the in-kernel copy and crashes
+	 * the target host. The Discovery controller is unauthenticated,
+	 * so the bug is reachable from any reachable fabric peer.
+	 */
+	if (offset > alloc_len) {
+		req->error_loc =
+			offsetof(struct nvme_get_log_page_command, lpo);
+		status = NVME_SC_INVALID_FIELD | NVME_STATUS_DNR;
+		goto out_free_buffer;
+	}
+
+	copy_len = min_t(size_t, data_len, alloc_len - offset);
+	status = nvmet_copy_to_sgl(req, 0, buffer + offset, copy_len);
+	if (!status && copy_len < data_len)
+		status = nvmet_zero_sgl(req, copy_len, data_len - copy_len);
+out_free_buffer:
 	kfree(buffer);
 out:
 	nvmet_req_complete(req, status);
-- 
2.43.0