From 0ae5ea00cc511110733665b2935a154d5e569480 Mon Sep 17 00:00:00 2001 From: Frediano Ziglio Date: Fri, 27 Mar 2026 14:13:38 +0100 Subject: [PATCH] Buffer overflow in drivers/xen/sys-hypervisor.c The build id returned by HYPERVISOR_xen_version(XENVER_build_id) is neither NUL terminated nor a string. The first causes a buffer overflow as sprintf in buildid_show will read and copy till it finds a NUL. 00000000 f4 91 51 f4 dd 38 9e 9d 65 47 52 eb 10 71 db 50 |..Q..8..eGR..q.P| 00000010 b9 a8 01 42 6f 2e 32 |...Bo.2| 00000017 So use a memcpy instead of sprintf to have the correct value: 00000000 f4 91 51 f4 dd 00 9e 9d 65 47 52 eb 10 71 db 50 |..Q.....eGR..q.P| 00000010 b9 a8 01 42 |...B| 00000014 (the above have a hack to embed a zero inside and check it's returned correctly). This is XSA-485 / CVE-2026-31786 Fixes: 84b7625728ea ("xen: add sysfs node for hypervisor build id") Signed-off-by: Frediano Ziglio Reviewed-by: Juergen Gross Signed-off-by: Juergen Gross --- drivers/xen/sys-hypervisor.c | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-) diff --git a/drivers/xen/sys-hypervisor.c b/drivers/xen/sys-hypervisor.c index b1bb01ba82f8..91923242a5ae 100644 --- a/drivers/xen/sys-hypervisor.c +++ b/drivers/xen/sys-hypervisor.c @@ -366,6 +366,8 @@ static ssize_t buildid_show(struct hyp_sysfs_attr *attr, char *buffer) ret = sprintf(buffer, ""); return ret; } + if (ret > PAGE_SIZE) + return -ENOSPC; buildid = kmalloc(sizeof(*buildid) + ret, GFP_KERNEL); if (!buildid) @@ -373,8 +375,10 @@ static ssize_t buildid_show(struct hyp_sysfs_attr *attr, char *buffer) buildid->len = ret; ret = HYPERVISOR_xen_version(XENVER_build_id, buildid); - if (ret > 0) - ret = sprintf(buffer, "%s", buildid->buf); + if (ret > 0) { + /* Build id is binary, not a string. */ + memcpy(buffer, buildid->buf, ret); + } kfree(buildid); return ret; -- 2.53.0