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55bd5e685b
drm_dp_sideband_append_payload() has three related bugs when processing
device-provided sideband reply data:
1. Zero-length curchunk_len underflow: msg_len is a 6-bit field taken
directly from the DP sideband header. If a device sends msg_len=0,
curchunk_len is set to zero. The condition (curchunk_idx >= curchunk_len)
is immediately true, and curchunk_len-1 wraps to 255 (u8 underflow).
drm_dp_msg_data_crc4() reads 255 bytes from chunk[48], then memcpy()
writes 255 bytes into msg[], both far out of bounds.
2. chunk[48] overflow: curchunk_len can reach 63 (6-bit field). chunk[] is
only 48 bytes. Multi-iteration payload assembly appends 16-byte blocks
until curchunk_idx reaches curchunk_len, writing up to 15 bytes past
the end of chunk[] into msg[].
3. msg[256] overflow: each chunk contributes (curchunk_len-1) bytes to
msg[]. No check ensures curlen + (curchunk_len-1) stays within msg[256],
so the memcpy can spill into adjacent struct fields.
All three are reachable from any DP MST device that can forge sideband
reply messages on a physical connection.
Fixes: ad7f8a1f9c ("drm/helper: add Displayport multi-stream helper (v0.6)")
Cc: <stable@vger.kernel.org> # v3.17+
Signed-off-by: Ashutosh Desai <ashutoshdesai993@gmail.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patch.msgid.link/20260410041901.2438960-1-ashutoshdesai993@gmail.com
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