[PATCH 1/2] linux-user: Fix zero_bss for RX PT_LOAD segments

[Helge Deller <deller@kernel.org>]

From: Razvan Ghiorghe <razvanghiorghe16@gmail.com>

zero_bss() incorrectly assumed that any PT_LOAD containing .bss must be
writable, rejecting valid ELF binaries where .bss overlaps the tail of
an RX file-backed page.

Instead of failing, temporarily enable write access on the overlapping
page to zero the fractional bss range, then restore the original page
permissions once initialization is complete.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/3179
Signed-off-by: Razvan Ghiorghe <razvanghiorghe16@gmail.com>
Reviewed-by: Helge Deller <deller@gmx.de>
Signed-off-by: Helge Deller <deller@gmx.de>
---
 linux-user/elfload.c | 37 +++++++++++++++++++++++--------------
 1 file changed, 23 insertions(+), 14 deletions(-)

diff --git a/linux-user/elfload.c b/linux-user/elfload.c
index 35471c0c9a..59b543f740 100644
--- a/linux-user/elfload.c
+++ b/linux-user/elfload.c
@@ -449,12 +449,6 @@ static bool zero_bss(abi_ulong start_bss, abi_ulong end_bss,
 {
     abi_ulong align_bss;
 
-    /* We only expect writable bss; the code segment shouldn't need this. */
-    if (!(prot & PROT_WRITE)) {
-        error_setg(errp, "PT_LOAD with non-writable bss");
-        return false;
-    }
-
     align_bss = TARGET_PAGE_ALIGN(start_bss);
     end_bss = TARGET_PAGE_ALIGN(end_bss);
 
@@ -472,20 +466,35 @@ static bool zero_bss(abi_ulong start_bss, abi_ulong end_bss,
              */
             align_bss -= TARGET_PAGE_SIZE;
         } else {
+            abi_ulong start_page_aligned = start_bss & TARGET_PAGE_MASK;
             /*
-             * The start of the bss shares a page with something.
-             * The only thing that we expect is the data section,
-             * which would already be marked writable.
-             * Overlapping the RX code segment seems malformed.
+             * The logical OR between flags and PAGE_WRITE works because
+             * in include/exec/page-protection.h they are defined as PROT_*
+             * values, matching mprotect().
+             * Temporarily enable write access to zero the fractional bss.
+             * target_mprotect() handles TB invalidation if needed.
              */
             if (!(flags & PAGE_WRITE)) {
-                error_setg(errp, "PT_LOAD with bss overlapping "
-                           "non-writable page");
-                return false;
+                if (target_mprotect(start_page_aligned,
+                                    TARGET_PAGE_SIZE,
+                                    prot | PAGE_WRITE) == -1) {
+                    error_setg_errno(errp, errno,
+                                    "Error enabling write access for bss");
+                    return false;
+                }
             }
 
-            /* The page is already mapped and writable. */
+            /* The page is already mapped and now guaranteed writable. */
             memset(g2h_untagged(start_bss), 0, align_bss - start_bss);
+
+            if (!(flags & PAGE_WRITE)) {
+                if (target_mprotect(start_page_aligned,
+                                    TARGET_PAGE_SIZE, prot) == -1) {
+                    error_setg_errno(errp, errno,
+                                    "Error restoring bss first permissions");
+                    return false;
+                }
+            }
         }
     }
 
-- 
2.53.0