diff --git a/sys/arm/arm/busdma_machdep.c b/sys/arm/arm/busdma_machdep.c
--- a/sys/arm/arm/busdma_machdep.c
+++ b/sys/arm/arm/busdma_machdep.c
@@ -1035,7 +1035,7 @@
 		    segp))
 			break;
 		vaddr += sgsize;
-		buflen -= sgsize;
+		buflen -= MIN(sgsize, buflen); /* avoid underflow */
 	}
 
 cleanup:
diff --git a/sys/arm64/arm64/busdma_bounce.c b/sys/arm64/arm64/busdma_bounce.c
--- a/sys/arm64/arm64/busdma_bounce.c
+++ b/sys/arm64/arm64/busdma_bounce.c
@@ -898,7 +898,7 @@
 		    segp))
 			break;
 		vaddr += sgsize;
-		buflen -= sgsize;
+		buflen -= MIN(sgsize, buflen); /* avoid underflow */
 	}
 
 	/*
diff --git a/sys/powerpc/powerpc/busdma_machdep.c b/sys/powerpc/powerpc/busdma_machdep.c
--- a/sys/powerpc/powerpc/busdma_machdep.c
+++ b/sys/powerpc/powerpc/busdma_machdep.c
@@ -656,7 +656,7 @@
 		    segp))
 			break;
 		vaddr += sgsize;
-		buflen -= sgsize;
+		buflen -= MIN(sgsize, buflen); /* avoid underflow */
 	}
 
 	/*
diff --git a/sys/riscv/riscv/busdma_bounce.c b/sys/riscv/riscv/busdma_bounce.c
--- a/sys/riscv/riscv/busdma_bounce.c
+++ b/sys/riscv/riscv/busdma_bounce.c
@@ -705,7 +705,7 @@
 		    segp))
 			break;
 		vaddr += sgsize;
-		buflen -= sgsize;
+		buflen -= MIN(sgsize, buflen); /* avoid underflow */
 	}
 
 cleanup:
diff --git a/sys/x86/x86/busdma_bounce.c b/sys/x86/x86/busdma_bounce.c
--- a/sys/x86/x86/busdma_bounce.c
+++ b/sys/x86/x86/busdma_bounce.c
@@ -733,7 +733,7 @@
 		    segp))
 			break;
 		vaddr += sgsize;
-		buflen -= sgsize;
+		buflen -= MIN(sgsize, buflen); /* avoid underflow */
 	}
 
 	/*
@@ -808,7 +808,7 @@
 			break;
 		KASSERT(buflen >= sgsize,
 		    ("Segment length overruns original buffer"));
-		buflen -= sgsize;
+		buflen -= MIN(sgsize, buflen); /* avoid underflow */
 		if (((ma_offs + sgsize) & ~PAGE_MASK) != 0)
 			page_index++;
 		ma_offs = (ma_offs + sgsize) & PAGE_MASK;