[openssh-commits] [openssh] 03/87: disallow non-MAP_ANON mmap(2) in seccomp sandbox

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djm pushed a commit to branch master
in repository openssh.

commit 46ea12755813564424ff7a3096ed6e029fbaa9b7
Author: Damien Miller <djm at mindrot.org>
AuthorDate: Fri Oct 30 13:05:03 2026 +1100

    disallow non-MAP_ANON mmap(2) in seccomp sandbox
    
    harden sandbox by disallowing non-anonymous mappings
    
    Anthropic ANT-2026-DVPHBUJ2
---
 sandbox-seccomp-filter.c | 26 +++++++++++++++++++++++++-
 1 file changed, 25 insertions(+), 1 deletion(-)

diff --git a/sandbox-seccomp-filter.c b/sandbox-seccomp-filter.c
index 9454f18ee..2d83fc3e6 100644
--- a/sandbox-seccomp-filter.c
+++ b/sandbox-seccomp-filter.c
@@ -154,6 +154,25 @@
 	/* reload syscall number; all rules expect it in accumulator */ \
 	BPF_STMT(BPF_LD+BPF_W+BPF_ABS, \
 		offsetof(struct seccomp_data, nr))
+/* Deny unless syscall argument contains all values in mask */
+#define SC_DENY_UNLESS_ARG_BITS(_nr, _arg_nr, _arg_mask, _errno) \
+	BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (_nr), 0, 8), \
+	/* load, mask and test syscall argument, low word */ \
+	BPF_STMT(BPF_LD+BPF_W+BPF_ABS, \
+	    offsetof(struct seccomp_data, args[(_arg_nr)]) + ARG_LO_OFFSET), \
+	BPF_STMT(BPF_ALU+BPF_AND+BPF_K, ((_arg_mask) & 0xFFFFFFFF)), \
+	BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, ((_arg_mask) & 0xFFFFFFFF), 0, 3), \
+	/* load, mask and test syscall argument, high word */ \
+	BPF_STMT(BPF_LD+BPF_W+BPF_ABS, \
+	    offsetof(struct seccomp_data, args[(_arg_nr)]) + ARG_HI_OFFSET), \
+	BPF_STMT(BPF_ALU+BPF_AND+BPF_K, \
+	    (((uint32_t)((uint64_t)(_arg_mask) >> 32)) & 0xFFFFFFFF)), \
+	BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, \
+	    (((uint32_t)((uint64_t)(_arg_mask) >> 32)) & 0xFFFFFFFF), 1, 0), \
+	BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ERRNO|(_errno)), \
+	/* reload syscall number; all rules expect it in accumulator */ \
+	BPF_STMT(BPF_LD+BPF_W+BPF_ABS, \
+		offsetof(struct seccomp_data, nr))
 #define SC_DENY_UNLESS_MASK(_nr, _arg_nr, _arg_val, _errno) \
 /* Special handling for futex(2) that combines a bitmap and operation number */
 #if defined(__NR_futex) || defined(__NR_futex_time64)
@@ -196,9 +215,14 @@
 # else
 #  define SC_MMAP_FLAGS MAP_PRIVATE|MAP_ANONYMOUS|MAP_FIXED
 # endif /* MAP_FIXED_NOREPLACE */
-/* Use this for both __NR_mmap and __NR_mmap2 variants */
+/*
+ * Use this for both __NR_mmap and __NR_mmap2 variants.
+ * Only anonymous mappings are needed; requiring MAP_ANONYMOUS prevents
+ * mapping of any file descriptor the process holds (e.g. the client socket).
+ */
 # define SC_MMAP(_nr) \
 	SC_DENY_UNLESS_ARG_MASK(_nr, 3, SC_MMAP_FLAGS, EINVAL), \
+	SC_DENY_UNLESS_ARG_BITS(_nr, 3, MAP_ANONYMOUS, EINVAL), \
 	SC_ALLOW_ARG_MASK(_nr, 2, PROT_READ|PROT_WRITE|PROT_NONE)
 #endif /* __NR_mmap || __NR_mmap2 */
 

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